 Consider our galaxy, galaxy A, separated by a large distance from galaxy B. In galaxy A's frame of reference, it is at rest. Galaxy B is moving away. Its distance from the Milky Way will continue to increase as time goes on. It follows that going backwards in time, galaxy B was getting closer to the Milky Way. We see that, at some point in the distant past, they would have been extremely close to each other. Assuming for now that the velocity is constant, we can divide it into the current distance between the two galaxies to see how long it took them to get this far apart. That's just one over the Hubble constant. So, without actually knowing the distance between them or their separation velocity, we find the two galaxies would have taken 14 billion years to reach their current separation. When we combine the cosmological principle with our view that all distant galaxies are moving away from us, we conclude that all galaxies are moving away from each other and that the further away a galaxy is from any other galaxy, the faster it is moving away. It follows that going back in time, all galaxies in the universe were once extremely close together, if not actually in the exact same place at the exact same time 14 billion years ago.