One theory which dominates the scientific society is the big bang theory.

The Big Bang theory is the most acceptable theory today about the creation of the universe. According to this theory the universe was created in a big bang about 12-15 billion years ago. Science has no way of predicting what was before that time. Before the big bang all the matter in the universe as we know it today was dense in one point. It's hard to imagine but the universe was that point, there was nothing else BUT that point. Then a big blast occurred and the point started to expand with tremendous speed. All the material init moved away from the blast into space . The explosion also created alot of radiation. Evidence for this is the Cosmic background radiation as we see it today. Since then, that point which was the early universe has expanded to the size we know today, and is continuing to expand. The evidence for this is the measurement of the galaxies seem to get further apart as we measure it today.

Immediately after the big bang the universe was filled with a dense "soup" of subatomic particles which are called quarks and leptons (such as electrons) and their antiparticle equivalents. By 0.01 second after the big bang some of the quarks had united to form neutrons and protons. After another 2 seconds the only leptons remaining were electrons; the antiparticles had been annihilated. After 3.5 minutes the hydrogen and helium nuclei had formed. After a million years the universe was populated with hydrogen and helium atoms, the raw material of stars and galaxies. The initial radiation from the big bang has grown less energetic. This "cosmic background radiation" today has a temperature equivalent to 3 degrees above absolute zero which is about -270 C.

Either it will become a Closed universe where the universe will encounter a critical point were the gravity between galaxies will overcome the speed given to them from the big bang and the universe will start to shrink. All the galaxies will move to a certain point in space where they will be one highly concentrated point like the universe was like before the big bang. That tragic death of the universe was named by some scientists as "The Big Crunch". Then the process may start all over again creating a new universe (it might have also occured before our present universe).

Or it will become a flat universe where the universe will continue it's expansion, but the rate of expansion will decrease nearing zero but never below it.

Or it will become a open universe where the universe will continue expanding endlessly.

Which one of these options will become the destiny of our universe is up for discussions in the scientific society today, and no common agreement has been reached, however we are still in the expanding phase.