Gravitational Potential Energy

Graphic - grav
Andromeda M31 Source

The universe has a very large (negative) gravitational potential energy ..... too large in fact.

Estimates of the visible mass of the universe are too low to account for the current structure of the universe. Therefore there must be lots of dark matter out there somewhere, but in what form? One of the current theories is that a weakly interacting particles called neutrinos have non-zero mass. If this is the case, particle physicists will have to completely rewrite their theories of the universe.

The amount of dark matter out there determines the fate of our universe. Will it expand for ever, and die a slow death, or contract into a big crunch?