# Dark energy vs dark matter

I would like to know the difference between dark energy and dark matter. The way i understand it they are both detected by the effect the have on other ojbects. So why are they different?
Thanks

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That "dark" has been used to describe them should not be taken to mean they are related. The term dark matter is being used to describe matter that is known to exist despite knowledge of what it actually consists of. Dark energy is a far more vague idea. It is more or less just a place holder for whatever is causing the expansion of the universe to accelerate.

Redbelly98
Staff Emeritus
Homework Helper
We know about dark matter because of it's gravitational effect on other matter within a galaxy. Rebound explained dark energy (what we know about it) pretty well.
I would like to know the difference between dark energy and dark matter. The way i understand it they are both detected by the effect the have on other ojbects. So why are they different?
Thanks
Well, everything is detected by the effect it has on other objects.

Thank you for responding, but my question still remains. I realize it it believed that dark matter is believed for keeping things in their place. But why could not the same thing be said for dark matter also causing other things ie the universe to expand? There must be something to it or why would a group of scientists be working in an abandoned mine 2500 or so feet underground trying to catch dark matter. Isn't the universe wonderful.
Thanks again

I believe dark energy has to do with the conservation of energy within the entire universe as it expands. Or it could account for the mass amount of energy that physicists come up with when they do certain calculations about the amount of energy in the universe.

I have a very vague understanding of these topics, however, so please dont quote me on it o.o

Redbelly98
Staff Emeritus
Homework Helper
Thank you for responding, but my question still remains. I realize it it believed that dark matter is believed for keeping things in their place. But why could not the same thing be said for dark matter also causing other things ie the universe to expand?
Well, you would have to provide evidence that dark matter and dark energy are the same thing. Lacking that evidence, we assume they are different until it is shown otherwise. What has been observed of them are two distinct, different, and contrary effects: attraction in the case of dark matter, and repulsion in the case of dark energy.
There must be something to it or why would a group of scientists be working in an abandoned mine 2500 or so feet underground trying to catch dark matter.
There is certainly something to dark matter, I never said otherwise. Scientists are trying to figure out how it fits into the Standard Model of known particles.

Thank you for responding, but my question still remains. I realize it it believed that dark matter is believed for keeping things in their place. But why could not the same thing be said for dark matter also causing other things ie the universe to expand? There must be something to it or why would a group of scientists be working in an abandoned mine 2500 or so feet underground trying to catch dark matter. Isn't the universe wonderful.
Thanks again
The main thing is this:
Dark matter is called matter because it is affected in the same way as other stuff by gravity. No matter that we know of has any of the properties associated with dark energy. You may say, well if we don't know what dark matter really consists of how do we know it doesn't have these sorts of properties? The answer is we don't, but in the absence of evidence there is no reason to attribute the expansion of the universe to it. On the other hand, by virtue of the fact that it is apparently affected by gravity, it is nonsensical to suggest it is also responsible for anti-gravity. I.e. dark energy, the repulsive force that is thought to be causing the expansion of spacetime, is often thought of as opposing gravity.

Again, you could just as easily call dark matter "globs of stuff" and dark energy "force field." Two completely different things as much as magnetism is different from a table.

To use the word nonsensical in any matter regarding science is inappropriate. Galliao said the earth revolved around the sun and was kicked out of the church and placed under house arrest for the rest of his life. Guess what he was right. We and our science are infants in the universe. We know little and have very much to learn. No theory is silly or whatever until there is absolute proof. An open mind and questions are the best ways to learn. Thanks again for the input.

Chronos
Gold Member
Dark energy is an effective theory that may be related to the 'inflation' believed to have immediately followed the big bang [inflation is also an effective theory]. We do not know what caused inflation, or why it 'shut off'. We currently observe a universe that apparently started to 'inflate' again about 5 billion years ago, albeit much more slowly than in the earlier inflationary epoch. The supernova legacy study [Perlmutter, et. al.] is the principle observational evidence, and is fairly compelling. Like inflation, it solves a number of other hitherto unexplained mysteries of the universe. Scientists are reluctant to accept 'effective theories' until reasonable alternatives are excluded. We go with what works until something better comes along.

I would like to know the difference between dark energy and dark matter. The way i understand it they are both detected by the effect the have on other ojbects. So why are they different?
Thanks
you see,dark matter IS matter.actually, dkm is an elusive particle called the neutrino.vast quantittttttttttttttttes of it was produced during the big bang. it is responsible for holding galaxies together.it creates gravity,on the other hand,dark energy is the mysterious enrgy caused by the continual production and destruction of matter-antimatter pairs.it is responsible for accelerating the rate of expansion of the universe.it acts opposite to gravity.got it!!

you see,dark matter IS matter.actually, dkm is an elusive particle called the neutrino.vast quantittttttttttttttttes of it was produced during the big bang. it is responsible for holding galaxies together.it creates gravity,on the other hand,dark energy is the mysterious enrgy caused by the continual production and destruction of matter-antimatter pairs.it is responsible for accelerating the rate of expansion of the universe.it acts opposite to gravity.got it!!
Dark matter is absolutely, positively, demonstrably not neutrinos. Neutrinos are extraordinarily light and this prevents them from being bound gravitationally to systems that aren't extremely large, given that the overall temperature of the universe is, and always has been, non-zero. Put simply, they aren't massive to have created the large-scale structure of the universe at the time we know it existed. Dark matter is something else.

Additionally, dark matter doesn't "create" gravity any more than electrons create electromagnetism. Electrons are a source of electromagnetic forces and dark matter is a source of gravitational forces.

To use the word nonsensical in any matter regarding science is inappropriate. Galliao said the earth revolved around the sun and was kicked out of the church and placed under house arrest for the rest of his life. Guess what he was right. We and our science are infants in the universe. We know little and have very much to learn. No theory is silly or whatever until there is absolute proof. An open mind and questions are the best ways to learn. Thanks again for the input.
I disagree. In any event, I believe you took the expression a little more to heart than I intended. It was not intended as an insult, but rather to indicate the 2 ideas are self-contradictory.

Ich
Melodie Ann said:
To use the word nonsensical in any matter regarding science is inappropriate.
That doesn't make sense.
Science is the attempt to make at least some sense of the world we live in. I can't see how this could be done without weeding out the nonsensical explanations.
No theory is silly or whatever until there is absolute proof.
There's no such thing as absolute proof in science. But there's a plethora of silly theories in fringe science.

so is there a relation between dark matter and gravitons.

Thank you for responding, but my question still remains. I realize it it believed that dark matter is believed for keeping things in their place. But why could not the same thing be said for dark matter also causing other things ie the universe to expand? There must be something to it or why would a group of scientists be working in an abandoned mine 2500 or so feet underground trying to catch dark matter. Isn't the universe wonderful.
Thanks again
In expansion of universe, quantities of ordinary matters and dark matter keep constant. They are diluted more and more as volume of universe increases.
But dark energy increase according to volume of the universe. Or its density remains constant. Dark energy is energy that vacuum space has. Every part of space both long existing one and new generated one by inflation of universe have same dark energy density.
Regards.

Just call Dark Energy the Cosmological (Expansion) Constant ($$\Lambda$$), it resists the confusion and it is more relative to describing its function. There is a relation with everything that has mass and energy to gravitons. Because anything with mass and energy emits gravitons (hypothetically) that tell space-time how to warp via the Stress-Energy Tensor I believe.

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Additionally, dark matter doesn't "create" gravity any more than electrons create electromagnetism. Electrons are a source of electromagnetic forces and dark matter is a source of gravitational forces.[/QUOTE]

i meant if dark matter is the souce of gravitational forces does this mean that it is the gravitons. what i want to ask is the mediators of electromagnetism are photons but the source is electrons ,is it that the same relation with gravity is it sourced by dark matter with gravitons as mediators.

i meant if dark matter is the souce of gravitational forces does this mean that it is the gravitons. what i want to ask is the mediators of electromagnetism are photons but the source is electrons ,is it that the same relation with gravity is it sourced by dark matter with gravitons as mediators.
Anything that has energy (strictly, anything that has a non-zero stress-energy tensor) acts as a source of gravity. So, dark matter is a gravitational source, as is ordinary matter, light, dark energy, etc.

It's less clear that it's consistent to think of gravity as being mediated by gravitons in the same sense that EM forces are mediated by photons. The problem is that the EM field equations are linear, allowing superpositions of different solutions, while the Einstein field equations of GR are highly non-linear.

There is certainly something to dark matter, I never said otherwise.
I will.

What is known is that given the velocity of the stars in a galaxy the stars in the galaxy should be headed outward from the center. Given the age of galaxies, over time the galaxy would expand until it was no longer a galaxy, which is not in keeping with the physical evidence.

What prevents (has prevented) the expansion is not known.

As gravity is a known confining force, One possible explanation, is that there is additional unseen form of matter (termed dark matter) that contributes to the visible matter gravitational field resulting in a total gravitational force sufficient to keep the stars in orbit around the center of the galaxy.

Dark matter is a guess at a solution.

The problem of how galaxies keep their stars in orbit is such a fundamental problem, that claiming dark matter is the cause is better than admitting the weakness of the guess known as dark matter.

Dark energy is also a guess to explain the measured (using the present cosmological model) accelerated expansion of the universe.

Scientists are trying to figure out how it fits into the Standard Model of known particles.
It doesn't.

Dark matter is absolutely, positively, demonstrably not neutrinos. Neutrinos are extraordinarily light and this prevents them from being bound gravitationally to systems that aren't extremely large, given that the overall temperature of the universe is, and always has been, non-zero. Put simply, they aren't massive to have created the large-scale structure of the universe at the time we know it existed. Dark matter is something else.

Additionally, dark matter doesn't "create" gravity any more than electrons create electromagnetism. Electrons are a source of electromagnetic forces and dark matter is a source of gravitational forces.
It's not just neutrinos but

http://astro.berkeley.edu/~mwhite/darkmatter/hdm.html

Neutrinos are probably a type of it though.

Hot dark matter was one of those good ideas that turned out to be wrong. Also that page is somewhat out of date. As we are getting more and more data, the possible amount of hot matter in the universe has dropped to the point that it's not significant.

Neutrinos are probably a type of it though.
There are a lot of different forms of "dark matter." But the dark matter that makes up most of the universe just can't be neutrinos as we currently understand them.

The problem is that the EM field equations are linear, allowing superpositions of different solutions, while the Einstein field equations of GR are highly non-linear.
Just to express this statement in another way.

You can think of EM as being made up of photons because you can take one photon study it. Take another photon, study that, and if you have two photons, what happens is more or less what happens when you add one photon to another one photon. If you have two photons, they aren't going to be exchanging too many photons with each other, and so you can think of two photons as the combination of two single photons (more or less).

GR doesn't work that way. The problem is that if you try to think of gravity as the exchange of gravitons, you run into big problems because those gravitons will start exchanging gravitons and those gravitons will start exchanging gravitons, and those gravitons will start exchanging gravitons etc. etc. etc.

What you end up with is a total mathematical mess, which makes you wonder if thinking about gravitons is the right way of thinking about the problem.

Hot dark matter was one of those good ideas that turned out to be wrong. Also that page is somewhat out of date. As we are getting more and more data, the possible amount of hot matter in the universe has dropped to the point that it's not significant.

There are a lot of different forms of "dark matter." But the dark matter that makes up most of the universe just can't be neutrinos as we currently understand them.
I didn't say neutrinos made up most of the dark matter in the universe, simply they are a type of dark matter.

Probably just taking this part out of context
Dark matter is absolutely, positively, demonstrably not neutrinos.