Find the opposite, vacuum expansion, or whatever name it goes under depending what theorist you ask, equally difficult to fathom, really. Maybe they're two faces of one coin, with one pole being local, near matter, and the other end distributed over the whole place, so the correlation is hard to see. Sometimes you really have to think laterally, with dimensions.

According to Einstein even a lightbeam of sufficient high energy would exert a gravitational field on its surroundings. Maybe there's the answer - energy. Remember mass is just an INCREDIBLE amount of energy locked in the M part of the equation he came up with.

The living are the dead on holiday. -- Maurice de Maeterlinck (1862-1949)