According to most scientific theories, the universe will not stay as we know it forever.
Most scientists agree that the universe is still expanding as a result of the initial force of the big bang. The differences of opinion on the future depend on how much physical matter exists in the universe.
If there is enough matter (and we don’t know just how much the critical amount is), the force of gravity inherent in each bit of matter will sooner or later slow and stop the outward expansion. Gravity will cause the matter to reverse its course and begin to contract. At some point, the universe will fall in upon itself.
Scientists disagree on what might happen if the universe collapses in what some call the “big crunch.” Another big bang might happen and the universe might begin all over again. (Would it develop in the same way again? No one knows.) A different conclusion has the big crunch creating a massive black hole, consuming time and space just as the big bang created them.
If there is not enough matter in the universe, then there is not enough gravity to reverse the course of expansion. The universe will continue to expand and the matter will continue to cool as it grows farther and farther apart. Stars will eventually die out, leaving all matter dark, cold, and essentially dead, though always expanding.
Remember, however, that some 15 billion years have passed since the big bang. The chances of someone being around to witness the big crunch or the death of the last star are the same as anyone having seen the big bang.