"Large" refers to stars, galaxies, clusters of galaxies, and one or more universes.

Relationships between large objects were initially described by Isaac Newton's classical theories of gravity.

At the start of the 20th century Albert Einstein reinterpreted the relationships in terms of relativity.

He developed two theories.

The Special Theory of Relativity says that the speed of light is fixed  and that time, mass, energy, and the speed of objects are relative to the observer.  

The General Theory of Relativity says that gravity happens when big, heavy objects distort the surrounding space-time.

Johnson's point is to show how we have constructed a framework for the cosmos based  on observations from one small planet.  We have extrapolated a universe from the evidence provided by billion-year old photons.

He wonders if there are not elements of scientific mythmaking in all this.