You’ve read about how originally there was nothing, and then suddenly….BOOM! And how the Universe started expanding, etc… But when you think about it… What’s really expanding? Is it the galaxies that are moving away from each other? Or is it the space between the galaxies that’s stretching, making galaxies move farther and farther from each other.

If you think about it, It could be either: moving galaxies or moving space. However one of the solutions breeds so much more results than the other. For example:
Nothing can move faster than light (300 000 000 km/s), so theoretically, if it were the galaxies that were moving, they wouldn’t be able to move away from each other faster than the speed of light. However, if it’s space that’s stretching, galaxies can move away from each other at a speed faster than light’s ! Remember that according to astronomer Edwin Hubble (The HST is named after him), the farther galaxies are from each other, the faster they move away from each other. So an 11 billion light-year distant galaxy could be getting away from us at one hell of a speed. It also allowed an unestimable gain of clarity, which is vital for any cosmologist or physicist. Using the stretching of space allowed physicists to explain a hive of phenomena which the first proposal couldn’t.