Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is dark matter/dark energy? Are there differences?

  1. Jan 13, 2016 #1
    Okay so I've been searching the internet for this answer, but I have not yet found it. What is dark matter/dark energy? What is the difference between the two? On a website that I was on earlier, someone described dark matter as a cluster of dark energy, which would make no sense because matter has mass and takes up space. I also learned that it's what holds galaxies together with gravity (theoretically), but energy has no mass so it cannot have a gravitational pull. Another thing I was wondering was if it is a solid, liquid, or gas? Or maybe something completely different? If dark matter makes up most of the universe... Why can't we find any? Thank you for answering this and sorry if I seem dumb.
  2. jcsd
  3. Jan 14, 2016 #2


    User Avatar
    Science Advisor
    Gold Member

    Did you really search the internet? The first page of google search has several reputable links (a NASA page on both of these, reasonable wikipedia articles on dark matter and dark energy) etc. Please try reading these, then come back with more specific questions.
  4. Jan 14, 2016 #3


    Staff: Mentor

    You're raising a number of issues that cover a fairly wide range; it might be better to focus on one at a time. But to address them as you state them:

    What website? Can you give a link? You're right that this statement makes no sense taken by itself (though not for the reason you give--see below); but without the context it's hard to know what the intent was.

    Dark energy also "has mass" in the relativistic sense; it has energy (see below). "Takes up space" is rather vague; dark energy has a well-defined density just like ordinary matter or dark matter. There are certainly properties that distinguish dark energy from dark matter and ordinary matter (see below), but lack of "mass" or "not taking up space" are not among them.

    Dark matter is hypothesized to be providing a good portion of the gravity needed to hold galaxies together. But dark energy plays no role in holding galaxies together.

    This is not correct. First, the distinction between "energy" and "mass" is not as clear-cut as you appear to think it is. It would be correct to say that some objects which have energy have no rest mass (like light); but I don't think that's what you meant (and having no rest mass does not mean having no gravitational pull--see below).

    The source of gravity in general relativity is not "mass"; it's the stress-energy tensor. Dark energy has a well-defined stress-energy tensor, so it can act as a source of gravity just like dark matter or ordinary matter. But its behavior as a source of gravity is different; it is a source of repulsive gravity, whereas dark matter and ordinary matter (and radiation, like light) are sources of attractive gravity. The property of being a source of repulsive gravity is why dark energy is thought to be the cause of the accelerating expansion of the universe; it's impossible to get such an effect with attractive gravity.

    Neither dark matter nor dark energy can be usefully categorized as any of these.

    This would be the best choice.

    Because it doesn't interact with anything except through its gravity (or if it does have any other interactions, they're so weak that we can't detect them). So we can tell that something producing gravity is there, but we have no other information about what it is.
  5. Feb 8, 2016 #4
    The first thing to realise is that you may not recognise everything that could be called ‘mass’ or ‘energy’. For example, an electric field has mass. The second thing to realise is that the terms mass and energy can be misunderstood, because they change their nature depending on the observer. For example, an asteroid has rest mass which is the mass someone standing on the asteroid would measure, but if we think of it as moving the mass is equal to the rest mass plus the kinetic energy. A photon has no rest mass, the mass of a photon is the mass of its kinetic energy.

    When we talk about objects in cosmology the description of mass becomes a bit more complicated again. We need to talk about the density of the matter, so a moving cloud of dust (say) gets more massive because it is moving, but it also gets more dense because length contraction means it takes up less space.

    There are other complications which I will glide over by assuming that we want to describe a point in space in a way that someone at that point would consider ‘natural’, meaning that we talk about 3 space dimensions, mutually orthogonal (ie at right angles), and an orthogonal (ie unbiased) time dimension.

    Given those assumptions, the point in space will have an Einstein tensor value that can be thought of as representing 10 Newtonian values. One will be the mass density, 3 will be the momentum density in each of the three directions in space, 3 will be the linear pressure (or tension) in each of the 3 directions, and the last 3 will be the twisting stress in each of the 3 directions.

    These values depend on how you view the point, but given all 10 values for one viewpoint, we could calculate what the values would be given some other viewpoint.

    When talking about space expanding or contracting, we are talking about a property of a large volume of space time: a small volume can always be described as expanding, contracting, or staying the same. In choosing a ‘natural’ way of describing space, I have in effect chosen to describe the small volume of interest as staying the same. However, it only stays the same for an instant, after which it might start to collapse or expand. The collapse we would ascribe to matter, both dark and non-dark. The expansion we would ascribe to dark energy. In fact, both may play a part, with one partly or wholly masking the other.

    So now I am able to say what is matter, and what is dark energy. Matter (in this sense) is mass, rest plus kinetic energy, plus pressure, dark energy is tension plus negative mass. In the early days it was assumed that the pressure or tension would be negligible and that negative mass could not exist, and the only thing that had a significant effect in our era is the mass. Einstein introduced a ‘cosmological constant’ which is a mixture of tension and mass (or of negative mass and pressure) when he thought he needed it to balance the tendency of the universe to expand or collapse. The mixture has the special property that makes it convenient to work with, and when you are guessing about something, you might as well guess something that it is convenient!

    Once the theory of the big bang origins became accepted, the cosmological constant became ignored. However, we again find that we can’t explain the rates of expansion and contraction of space. Galaxies generally spin, and gravity stops them from flying apart, but there does not seem to be enough mass in the galaxies to hold them together, so we guess there is a lot more mass there that we don’t know about, and we call that dark matter. On the other hand, the universe as a whole seems to be accelerating apart, so we guess that between the galaxies there is a lot of dark energy.

    Now as to why we can’t find any. First, although we think there is a lot of dark matter and dark energy in the universe, it is spread very thinly, unlike ordinary matter which clumps together to form stars and planets. Second, we have not found anything that can couple with the dark energy or dark matter. By couple, I mean exert a force on it. Third (really expanding on the second point) the dark energy and dark matter do not seem to absorb or emit light. The only effects we have been able to observe is the effect of its gravity (positive: dark matter, negative: dark energy) and gravitational lensing (for dark matter, in some special cases).
  6. Feb 9, 2016 #5


    User Avatar
    Gold Member

    Dark matter and dark energy are theories to explain why the stuff we see in telescopes doesn't behave the way that it should if Einstein's general relativity without a cosmological constant is the correct theory of gravity and most of the stuff in the universe is made up of stars and planets and black holes and cosmic gas and neutrinos in proportions similar to those in places we can observe directly. While it is possible to come up with theories that explain both dark energy and dark matter as having a common source, for all practical purposes, dark matter and dark energy are completely unrelated phenomena.

    Dark energy is one model for describing a mechanism by which observed acceleration of the universe could arise in a universe where the equations of general relativity lacked the cosmological constant, and it is equivalent to the limits of experimental data with a cosmological constant in the equations of general relativity. Basically, it is equivalent to an extremely faint field of energy that has a uniform constant density in all of empty space. It would not be a solid, liquid or gas, all of which are concepts applicable to matter. The closest equivalent to dark energy which is within the realm of your experience would be the Earth's magnetic field, but it would be much weaker and would interact only via gravity and not via any other force.

    Energy converted to matter equivalent via the formula E=mc^2 has the same gravitational pull as matter.

    Effects attributed to dark matter are easy to see and have been known for many decades. Dark matter is inferred to exist for a variety of reasons, the most obvious of which is that the observed speed at which the fringes of spiral galaxies rotate is inconsistent with any reasonable model of a galaxy made up mostly of stars with the observed distribution of stars within the galaxy moving in accordance with our best available theory of gravity called general relativity. The stars on the rim of spiral galaxies should fly off into space given the observed matter pulling on them at the speeds at which they are moving, but they don't. Many other tests of how much matter there is in a galaxy (such as gravitational lensing which is the amount by which the gravity of a galaxy bends light around it) also confirm that light bends far more than it should around galaxies if they galaxies were made mostly of stars. There are also more subtle effects of dark matter which are used to distinguish between different theories about what dark matter is like.

    We can estimate, based upon the movement of stars around galaxies and gravitational lensing, roughly how much matter we need in what kind of distribution to make galaxies behave the way that we observe them to behave. When we make this estimate, we predict a rugby ball shaped "halo" of dark matter around a typical spiral galaxy like ours that is much larger than the galaxy itself and much less disk-like and has little or no clumping, that has a very low density in any one place which we can estimate to an order of magnitude level in the vicinity of the solar system and how fast it should be moving.

    We know from trying to detect dark matter with sophisticated experiments, that if it exists, dark matter has virtually no interactions (other than via gravity) with any ordinary matter. Dark matter is the ultimate loner of the universe; it interacts with nothing through forces other than gravity. Like neutrinos (jillions of which fly through your body every moment of your life), it flies right though our bodies and even the entire planet Earth with virtually no contract. Indeed, it has to interact with ordinary matter even less often than neutrinos do, despite moving much more slowly than neutrinos do, and it can't interact very much with other dark matter either. Dark matter is sort of like an ultra-thin gas that can go right through anything, but even more diffuse and insubstantial. The fact there wouldn't be much dark matter in the solar system, and that it would be very evenly distributed, is why dark matter has no meaningful observable effects at the solar system scale. Dark matter has big effects only because it takes up a huge amount of space.

    But, in truth, we don't really know what dark matter is other than making some vague statements about the properties that it must have to produce what we observe. No particle collider has ever produced anything that is remotely a good fit to the known properties that dark matter must have to fit our observations.
  7. Feb 9, 2016 #6
    ... though by way of contrast, the earth's magnetic field exerts positive gravity (causing an accelerating contraction of space) whereas dark energy exerts negative gravity (causing an accelerating expansion of space).

    The term 'dark energy' is misleading (but we are stuck with it). This meaning of energy contrasts with the meaning of energy in the universally recognised identity E=MC^2.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook