I've read a little bit about cosmology, as I find it pretty interesting. Let's see if I can explain this without butchering the information too much...
First some background. You have to understand how light works to fully grasp this, but basically, whatever you see is actually a look into the past. For items very close to you which are illuminated only small distances away, this amount of time is negligible. But for those celestial bodies out in space that are lightyears away, what you are seeing is actually what the thing looked like that many years ago. For example, it takes something like 8 minutes for light from the Sun to reach the Earth. So, when you look at the Sun, you are actually viewing what it looked like 8 minutes ago.
This phenomenon is important when studying the universe. There is something called a supernova, which is what occurs when a star collapses on itself. When this happens, a massive amount of light and energy is released. This light travels for billions of lightyears. From this information, scientists can figure out the rate of expansion of the universe and the size as well. Don't ask me how exactly, but it's been done.
Something wasn't right with the calculations though. For some reason, the amount of matter and energy required to continually propel the universe's expansion against the force of gravity is many times the amount of matter and energy in the universe (Yes, somehow this sum can be calculated as well). Most thought this was an anomaly until something else came along as proof. It's called cosmic background radiation.
The cosmic microwave background (CMB) is the radiation that was released during an era of universal development called recombination, which is basically when the universe cooled down enough for atoms to begin forming. This radiation is the oldest light visible to scientists currently. Unfortunately, the CMB has dwindled to the point where the light is at microwave frequency. Everything emits microwaves, which is a very low energy light invisible to the naked eye, so it makes it very hard to read microwaves with all that "distortion" from everything else. Once scientists were able to "read the light," they further corroborated what they had calculated with the supernovas.
Once this was proven, a name was given to the unaccounted for matter and energy in the universe that fits into their equations. That is, dark matter (Non-illuminated matter) and dark energy (The force opposing gravity to cause universal expansion).
There is actually also something called exotic dark matter, but I don't really recall what that is.
Make sense at all?