The Shapley-Curtis debate in 1920 would be resolved, in favor of Curtis, in 1920, when it was discovered that this object:

M31: The Andromeda Galaxy

Was located at a distance of 2 million light years from us.

Hubble to the rescue:

Hubble detected Cepheid Variables in M31 and by using the Period-Luminosity Relation

Definitively showed that M31 had to be 2 million light years from us to make the Cepheids as dim as they appeared. Note that this required the existence of a 100-inch telescope to make this determination. So again, science and discovery is being advanced by better instrumentation.

But let's back up a bit now and return to 1909 when Einstein was thinking about the nature of gravity.

In the late 19th century, it was noticed that the observations of the position of Mercury did not agree with where Newtonian Mechanics said it should be. This means that Newtonian Mechanics is somehow an incomplete description.

But why? What is special about Mercury compared to the other planets?

Einstein postulated that gravity was the curvature of space time and this is the essence of General Relativity:

Space communicates with matter and instructs it how to move and, in turn, matter communicates with space and instructs it how to curve.

So for Mercury we have:

Precession of Mercury

Newton had assumed that space was flat

Okay, so how come we can observe the Universe? Next