What ia dark matter?

Think of light .. there is visible light and that which we cannot see. Same with matter, so the theory goes, there is the matter we can see everyday, then there is the matter we cannot see, which we call " Dark Matter." Even though, we cannot see it, we can see the effect it has, again so the theory goes, and from what I understand nothing is etched in stone yet, so the jury is still out on this one. Is it truly the effect from dark matter or is there something else going on, we'll have to stay tuned to find out.

Good question. Dark matter is the explanation astronomers use to explain the "missing mass" problem.

The problem goes as follows: we can measure the brightnesses and masses of galaxies through independent methods. When we calculate how much mass there needs to be to account for the measured brightness, however, we find that it is not enough to account for the measured mass. Hence there must be mass that we aren't seeing.

The leading explanation for this dark matter is that it is made up of fundamental particles (like electrons) that do not have charge or magnetic moments (unlike electrons) and so do not interact with photons. Thus we can't see them. Several candidate particles have been suggested by extensions to the Standard Model of Particle Physics (such as supersymmetry). We think that they interact through the Weak interaction, and several experiments are underway to try to detect these particles directly.

As for other types of matter, the most common one that we know is Baryonic matter. This includes protons, neutrons, and electrons (among some other more exotic particles) and hence all matter that we interact with on a regular basis.

You also mentioned antimatter. So the Standard Model says, every particle has an anti-particle. The anti-particle has the same mass but opposite charge as its counterpart. For instance, the anti-electron (aka the positron) has the same mass as the electron, but is positively charged. If a particle comes in contact with a corresponding anti-particle, they will annihilate each other and two gamma rays will be created, carrying the total energy given by E=mc^2. Antimatter is also considered to be Baryonic matter, however we don't believe there to be nearly as must antimatter in the Universe as there is regular matter.

Physical objects or particles that emit little or no detectable radiation of their own and are postulated to exist because of unexplained gravitational forces observed on other astronomical objects. Dark matter is believed to be part of the missing mass.