What is dark matter and dark energy, and how do we know they exist?

[HU742]

I have just completed my first physics class. I was wondering what dark matter and dark energy is? Also, if we can not see it how do we know that it is there? All answers would be appreciated.

 

[verty]

I have just completed my first physics class. I was wondering what dark matter and dark energy is? Also, if we can not see it how do we know that it is there? All answers would be appreciated.

Light bending around a galaxy measures the mass of the galaxy, but the intensity of the light measures the light matter (stellar mass if you like, but correct me if I'm wrong). These measurements differ because there is dark matter, matter we can't see.

Looking at wikipedia, it makes outrageous claims like the following one:

According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle.

But I think the original idea was just that it is matter we can't see from far away, sort of like how insect biomass is so great but insects are so small.

I may be wrong but this is the impression I got when I read about dark matter a few years ago.

 

[chrisbaird]

Dark matter is the gravitational glue that keeps galaxies from flying apart. Without dark matter, most galaxies would not have enough mass to create enough gravity to keep it in one piece. Dark matter gives enough extra mass to do the trick. Dark matter is not just dust or gas. You can see dust or gas in space when light shines through it. Dark matter is truly dark in the sense that there is not way to see it aside from it's gravitational effects. It must be some kind of matter that does not interact at all electromagnetically or through the nuclear forces, but does interact gravitationally.

Dark energy is what is causing the expansion of the universe to accelerate. There is even less known about dark energy as dark matter.

 

[verty]

Dark matter is truly dark in the sense that there is not way to see it aside from it's gravitational effects. It must be some kind of matter that does not interact at all electromagnetically or through the nuclear forces, but does interact gravitationally.

From the wikipedia page, I know it is not a great source:

Today, cold dark matter is the simplest explanation for most cosmological observations. "Cold" dark matter is dark matter composed of constituents with a free-streaming length much smaller than the ancestor of a galaxy-scale perturbation. This is currently the area of greatest interest for dark matter research...

The composition of the constituents of cold dark matter is currently unknown. Possibilities range from large objects like MACHOs (such as black holes[76]) or RAMBOs, to new particles like WIMPs and axions. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements.

I wanted to point out that brown dwarfs for example are included under the umbrella of dark matter.

 

[baudrunner]

The more answers that you read, the more ideas you will find on the subject of dark matter and dark energy. Since the physics community admits that it doesn't know what either DM or DE are, the arena is wide open to theorizing. DM and DE are inferred from observation. It may be quite possible that they don't exist, and that the observations are being misinterpreted and the theory misconstrued. For that matter, the acceleration of the Universe's expansion is probably an optical illusion, based on the energy losses incurred over the vast distance through which light is propagated. After all, one may say that if the amount of energy of a photonic wavelength at its point of reception, as in our retinas, is equal to its energy at its point of origin, as in a star many millions of light years away, then perpetual motion must also be possible, but you'll find nobody who agrees with that. It's a hot topic for debate. Where DM and DE are concerned, there are many who are reluctant to subscribe to the status quo, lest observation's interpretations are based on the optical illusion of the red shift being the result of an apparent acceleration of the Universe's expansion.

 

[verty]

The more answers that you read, the more ideas you will find on the subject of dark matter and dark energy. Since the physics community admits that it doesn't know what either DM or DE are, the arena is wide open to theorizing. DM and DE are inferred from observation. It may be quite possible that they don't exist, and that the observations are being misinterpreted and the theory misconstrued. For that matter, the acceleration of the Universe's expansion is probably an optical illusion, based on the energy losses incurred over the vast distance through which light is propagated. After all, one may say that if the amount of energy of a photonic wavelength at its point of reception, as in our retinas, is equal to its energy at its point of origin, as in a star many millions of light years away, then perpetual motion must also be possible, but you'll find nobody who agrees with that. It's a hot topic for debate. Where DM and DE are concerned, there are many who are reluctant to subscribe to the status quo, lest observation's interpretations are based on the optical illusion of the red shift being the result of an apparent acceleration of the Universe's expansion.

It makes sense to me that there should be matter in galaxies we can't see. One thing though, whatever there is in far away galaxies, there should be in our galaxy. Our galaxy should have dark matter too, even our solar system, it shouldn't be special. So I know where the idea comes from that dark matter is all around us unseen.

And if there are heuristic arguments that MACHO's can't account for the amount of dark matter needed to account for the observed lensing, I accept that too. I think it is a step to far to say that cosmologists agree that there must be an as yet undiscovered particle accounting for dark matter. It could of course happen that there is no other particle and lensing is just not how we thought it was.

 

[bapowell]

Looking at wikipedia, it makes outrageous claims like the following one:

There is nothing "outrageous" about this view -- it is the consensus understanding of modern cosmology. There are several avenues of evidence (e.g. the Bullet Cluster) that point towards a particulate form of dark matter (as opposed to modified gravity). You are correct that early observations of galaxy rotation curves indicated that there was some extra "dark" mass out there, but since that time we've learned much about the alternatives (like compact halo objects, planets, or other 'baryonic' sources of dark matter) and the best explanation is currently weakly interacting particles.

 

[Drakkith]

The more answers that you read, the more ideas you will find on the subject of dark matter and dark energy. Since the physics community admits that it doesn't know what either DM or DE are, the arena is wide open to theorizing.

I guess that depends on what you mean by "wide open to theorizing". Both dark matter and dark energy are severely constrained on what they might be by observations. Dark matter especially.

DM and DE are inferred from observation. It may be quite possible that they don't exist, and that the observations are being misinterpreted and the theory misconstrued.

Possible, but extremely unlikely. It's pretty hard to misinterpret the available evidence.

For that matter, the acceleration of the Universe's expansion is probably an optical illusion, based on the energy losses incurred over the vast distance through which light is propagated. After all, one may say that if the amount of energy of a photonic wavelength at its point of reception, as in our retinas, is equal to its energy at its point of origin, as in a star many millions of light years away, then perpetual motion must also be possible, but you'll find nobody who agrees with that.

This doesn't make any sense at all. And perpetual motion is known to be possible. Perpetual motion MACHINES are not, however.