If you are wondering what dark matter is, here is a good explanation from Alex Kritchevsky on Quora:

Dark matter...comes from the observation that galaxies should not be able to rotate at the speed they seem to rotate at based on their visible mass alone (and this is a huge effect - approximately 84% of the mass required for our observations seems to be invisible). It's 23% of the mass-energy total in the universe. If dark energy is 73% and dark matter is 23%, then there's only 4% left for the visible mass and energy in the universe, which is amazing.

So why is this important?

"Because the amount of matter and energy in the universe determines the rate of expansion," Dr. Michio Kaku tells us. "We now know there is a lot more dark energy than we previously thought. Therefore, the universe is undergoing an inflationary exponential expansion.  It is in a runaway mode, but here is the catch: we don't know how long that runaway mode is going to last."