It's something the astrophysicists made up because the Universe isn't acting the way their little theories say it should.
So they cooked up this 'dark matter' hokum.
L
Well, more generously (and I believe more accurately), it serves as a lesson in humility regarding the weakness of the syllogism. Pelion on Ossa, doncha know...
Galaxies are rotating too fast to fit the theory of angular momentum. Also, distant galaxies are expanding faster then they should according to the theory of relativity. Dark matter was theorized to help explain these discrepancies... Now there is something that is causing light to bend, but is not observable by either absorbing or reflecting energy. Since this behavior is neither like regular matter nor a black hole, and the fact that something is generating enough gravity* to cause light to bend they have dubbed it ‘Dark Matter’.
*Yes, I know that gravity is actually a warp in space-time. I am using it as scientific shorthand here.
The relevant theories are gravitational theory and field theory. They are theories, because they are supported by the evidence. In the case of dark matter, it was both a matter of field theory and gravitational theory, showing some particle(s) was missing observing something new, that fit the missing particle.
In gravitational theory, their was missing mass. Mass that must be there, but could not be observed as an otherwise interacting particle, by way of the electroM, weak, or strong force. In field theory, specifically with regard to strong interactions, there was a lack of explanation for the theory's prediction the charge*parity(CP) would be violated and the observation that CP is not violated experimentally. Parity is the symmetry of a particle(s) when it's direction is reversed. This strong CP problem can be understood by the existance of a particle called and axion. It has a very small mass, but does not interact through the strong, weak, or EM force. That's because, in an overly simple pic for this particle, all fields cancel due to symmetry and all that's left is mass. The particle was found over ~a year ago and reported on FR.
The axions would have been created shortly after the Big Bang, at the time the matter/antimatter imbalance was established and can explain both the CP problem and the missing mass required by GR.
"Galaxies are rotating too fast to fit the theory of angular momentum."
It's not that the spiral galaxies are rotating too fast, but the fact that the radial distribution of angular momentum is not what it should be, if all that was present is the visible mass. Instead of having a fast rotating core and a slow outer region, as would be expected from the distribution of what could be seen, the galaxies rotate with a high speed from the center out. The existance of massive, but otherwise non-interacting particles flying around the galaxy explains the motion. Axions fit.
"distant galaxies are expanding faster then they should according to the theory of relativity."
That's due to dark energy, not dark matter. Dark energy can be seen as the effect of the vacuum. It has a small negative gravitational constant. THe energy density of the universe is zero. It consists of mass and that contained in the EM, weak, and strong particle fields, which are denoted positive energy, and the negative(potential) energy of the gravitational field. The big bang doesn't consist of stuff flying out from and explosion. Space itself is expanding. As the space expands, the energy density decreases, and the relative effect of the vacuum increases. That means the gravitaitonal "constant" gets smaller, and a higher acceleration results.
"Now there is something that is causing light to bend, but is not observable by either absorbing or reflecting energy. Since this behavior is neither like regular matter nor a black hole, and the fact that something is generating enough gravity* to cause light to bend they have dubbed it ‘Dark Matter’.
In that way they IDed the dark matter in this case. Normally the dark matter flies at around the speed of light, but always below it. In spiral galaxies the trajectories for the axions can be calculated and they appear more concentrated at the outskirts. In this case the particles are all concentrated at the center and trajectories haven't been calculated. They 'll need to do that to explain the absence of observed angular momentum. Some questions are raised. The kinetic energy of those axions must have gone somewhere and gravity is qite a small force to accomplish the disipation of all that kinetic energy w/o resulting in a net angular momentum. they'll have to examine the possibilities of creating such a formation that way. If it's impossible to do it, they'll be a new discovery made in the physics involved.