And since there was already some notion about it from general relativity,

this was accepted right from the get-go.

Now, where does Hubble law come from?

So imagine space fill the galaxies, and the volumes stretch uniformly.

So this is the most multiplicative change, all right?

Every volume gets multiplied by some factor at one point,

whatever, over a period of time.

So that means that the increment over any given time interval

is going to be also proportional to the length that you're looking at.

So the increment of distance is proportional to the distance itself.

And since the same delta t for all galaxies you look at, then velocities will

be also proportional to the distance, and this is what Hubble's law says.

In fact, it's equivalent to the statement that if the space is uniform,

homogeneous, isotropic.

If you see this effect, that means the space expands and vice versa.