Okay, so most of you guys understand the "Big Bang Theory", right? This is my theory on why scientist believe that space is constantly expanding: Take my "water ballon theory" for example. Imagine a tiny water ballon that is so condensed with water that the water pressure inside is uncaculatible. Also, this water ballon is very unstable. Then all of a sudden, it explodes and water goes in every direction at once! (Note: There is no gravity) Like I said, the very condensed water (energy) pressure was uncaculatible so, the water continuously expands in every direction, like space.