First off, the matter of the universe was not created from the big bang, as somebody had said, the matter was already there, it simply exploded out into the universe. Secondly, it can not be proven that the mass of the universe remains constant, due to black wholes and wormholes which could theoretically allow time travel, allowing the universal mass at one point to be different from another. Matter can not be created or destroyed, true, but if wormholes allow time travel, it can couple, like if I send myself back in time 1 year, for 1 year there are 2 of me, and more matter in the universe than prior to that point. Or I could send myself forward one year and there would be no me at all for a year. So although I did not create nor destroy matter, there would be more at one point in time than another. Just wanted to get those 2 misconceptions out of the way.