Dark Matter:

One of the most exciting, and terrifying, ideas that exist at the present moment is the idea of dark matter and dark energy. When you look up at the night sky, you see billions of stars. What you don't see is the energy and matter forcing these galaxies apart. It's known that the universe is accelerating out in all directions. When you think of squeezing a balloon, you know that the pressure increases. When you pull the balloon apart without adjusting the amount of air in the balloon, you get a vacuum. Now, think of that balloon as our universe. Something must exist that is pulling the universe apart as well as something that tries to pull the universe back together. You can think of dark energy as your hands expanding the volume of the balloon causing everything in it to spread apart. Dark matter would be the vacuum forces that try to pull the balloon back to the right size. Dark energy and dark matter are actually contraction and repellent forces, but the action-reaction idea of trying to pull apart the balloon should give you an idea of what's happening in space. You can't directly see dark matter, but you can know of it's existence by examining the effects.

Now we reach the scary part. Matter does not change. If you stretch matter, all you can change is the density. As the universe expands, the dark matter becomes less dense. What doesn't change is energy. It is theorized that with an energy this high, the universe would expand so quickly that galaxies would disappear. Dark energy is a repellent force, so we know that it has a negative pressure. Using the Ideal Gas Law, PV=nRT, where n and R are constant in the universe, increasing volume and using a negative pressure, T must be significantly low. Many believe that space contains nothing, but there is matter in space. NASA predicts that there is approximately one atom per cubic centimeter. This means that the Ideal Gas Law could apply. Thus, all normal matter in existence would freeze and all life would cease to exist. The only way this can be prevented is if the dark matter of the universe evolves into a gravitational force that will slow the acceleration of the universe.

It is important to know that many in the science community do not believe that dark matter and dark energy exist. Many believe that the unexplained movement in the universe is just a property of space. After all, it is "theoretical" physics.

A video on dark matter can be found here.