Physicists believe that most matter, which is anything that has mass and takes up space, will eventually break apart because of radioactive decay. Even the most stable molecules and atoms will break into subatomic particles. Scientists do believe the universe is flat (or almost flat), so it will not collapse in on itself after a finite time. But if there really is an infinite amount of time, then even very unlikely events might happen, such as the formation of Boltzmann brains.