The mass of the universe, including the 'mass' equivalent of all the sources of energy ( light, neutrinos etc) produce their own gravitational fields. It is against this gravitational field that the universe works against during the expansion. So, the mass of the universe is trying to halt its own motion. A crude analogy would be if in a fireworks display, the individual particles exerted forces upon each other as they flew apart. If the force is great enough, the cloud would stop expanding. In general relativity the situation is slightly different, but heuristically, this fireworks example is not too bad so long as you do not take it too literally.