I honestly don't want this theory to prove correct because it's proof would mean that life is utterly and completely pointless. However, here it goes:
According to Einstein, matter cannot be created nor destroyed. it can only be changed. Same with energy, as far as I know. That being said, matter must have always existed. Which means that there was never a point where there was nothingness. However, most people say that time is linear, which means that everything had to begin at some point. However, something can't be born out of nothing. THis is the logical equivalent of 0 X 5, which still equals zero. Also, time has to be eternal also, since time can't just begin out of nothing either.
Now barring the existence of a god (the existence of God helps this theory, but I want to keep it strictly logical) and allowing that existence extends beyond our universe, it's easy to conclude that time flows in a loop. That allows matter to exist forever and energy to exist forever. That even helps with the big bang theory, previously a belief that pure nothingness just exploded into something.
(Sidenote: maybe our "big bang" wasn't the only big bang. Maybe it was one in a never-ending series of big bangs. That begs the question of whether this universe will die one day, or even collide with another universe.)