 In the late 1920s, Edwin Hubble discovered that except for a few nearby galaxies, all galaxies were moving away from us, and the further away they are, the faster they are moving. Along with the assumptions that there are no preferred places and no preferred directions in space, this means that all galaxies, not bound together by gravity, are moving away from each other. The flow of all galaxies away from each other, with faster velocities the further away from each other they are, cannot happen in a fixed volume because in a fixed volume some reference frames would have to have distant objects heading towards them for others to have them moving away. It can only be explained if the space that these galaxies exist in is itself expanding. Here's a one-dimensional example to illustrate why this is the case. Consider an 8 meter circle with marks 1 meter apart. If we are at the top mark and all the other marks are moving away from us, then, from other points of view, marks are getting closer. The system is not homogeneous, but if the apparent motion is due to the amount of space expanding, we get a different picture. Here the marks hold their position on the line, but the line grows. Let's say each meter on the line expands to 2 meters over the course of a minute. We see that the distance between adjacent marks goes up 1 meter and their apparent velocity, as seen by each other, is 1 meter per minute, but more distant marks have increased their distance and velocity by more than that. And the further away any two marks are, the more their distance and velocity have increased. And most importantly, this will be the same no matter which mark is used for the reference frame. In order to illustrate the point, this example uses an expansion rate that is 74,000 trillion times greater than the actual expansion rate as determined by the Hubble constant. The real expansion is very slow. If we take a look at what the expansion does to 1 meter, we see that it would take a million years to expand by just 7 millionths of a meter. That's way too slow to ever notice or even measure in a lab in a lifetime. And it is why it's so easy to overcome it with local gravity out to the Andromeda galaxy. It should be noted that this expansion of space itself does not pull apart objects that exist in that space. A meter stick does not expand. That's because the size of the meter stick is determined by the forces that hold it together and these forces are not changing.