Gravity and Dark Matter

Dark Matter is something that has no definition yet as no one knows what it is. Basically, it is a something-or-other that is probably more powerful than gravity because it is expanding the Universe faster and faster, while also never getting weaker.

If you look at our solar system, you will see that the Sun's gravity has less effect on the planet Uranus than it does our Earth, therefore Earth moves at a faster rate around the sun. But, if you were to look at the entire galaxy, our solar system travels around the center (where the Supermassive Black Hole is) at the same rate as every other system in the Milky Way. That means something is keeping everything together but it is impossible to detect what that something is (or at least we haven't yet).

At Space.com, they have an article about how our probes were expected to travel faster the further away from the sun they got (because gravity wasn't holding it back as it moved in the opposite direction), but they haven't. Pretty interesting read.

I read a book recently that had a chapter on Dark Matter which is where I got most of the above information.