The first amazing fact about gravitation is that the ratio of inertial mass to gravitational mass is constant wherever we have checked it. The second amazing thing about gravitation is how weak it is.

We get the exciting result that the total energy of the universe is zero. Why this should be so is one of the great mysteries - and therefore one of the important questions of physics. After all, what would be the use of studying physics if the mysteries were not the most important things to investigate?

The universe is very large, and its boundaries are not known very well, but it is still possible to define some kind of a radius to be associated with it.

Richard P. Feynman Quotes