How does the amount of matter in the universe affect the possible futures of the universe?

Answer 1

That's a wide-open question - literally.

Gravity pulls everything together. The Big Bang, we think, blasted everything apart. Which one will win? Will the universe keep expanding without limit, or will gravity, the only force that really reaches as far as we can imagine, eventually pull everything back together? We don't know. IF there is enough mass, it might be enough to slow down the expansion and bring everything back together in a "big crunch". If there isn't, then it might expand "forever".

The key point to remember is that we humans only know the tiniest sliver of a fraction of one percent of how the universe works. We've never been outside our own gravity well, we've barely gotten outside our atmosphere. We have two very old space probes that are only now approaching what we might call the boundary between the neighborhood of our star and venture into the vastness of interstellar space. Any time a scientist explains how the universe works, we can be certain that he is speaking from utter and complete ignorance, because we don't know.

We have theories, possibilities, guesses - and every physicist has his own, usually contradictory, guesses.

Answer 2

The amount of matter in the universe affects its future through gravity. More matter means stronger gravitational pull, which can determine the ultimate fate of the universe - whether it will continue expanding forever, eventually stop and contract, or reach a balance between expansion and contraction. The distribution of matter also influences the formation of structures like galaxies and galaxy clusters.