Science assumes that the universe started with a singularity, where there was nothing..nothing at all. Then logically, the universe is supposed to come back to its original state at some point of time..who knows. I have tried but could never understand 'quantum Fluctuations' which is said to be the reason for the big bang out of a singularity.

Further, the 'anti-matter' and 'negetive energies' are still theoretical concepts. If the same are proven someday, it would further help us to understand if the universe will die at all.

But, at present, it is a general opinion that universe will keep on expanding and may freeze out when all stars would become either black holes, neutron stars or white dwarfs.