In sub-atomic physics, an electron can be in several places at the same time, and its position is only fixed in one place when it is observed -- or so goes the theory.
Imagine that we place a living cat into a steel chamber, along with a device containing a vial of hydrocyanic acid, and a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a hammer, which will, in turn, break the vial and kill the cat. The observer cannot know whether or not an atom of the substance has decayed, and consequently, cannot know whether or not the cat has been killed. According to quantum law, the observation or measurement of sub-atomic particles affects an outcome, so that the outcome as such does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.) We know that this actually occurs at the subatomic level, because there are observable effects of interference, in which a single particle is demonstrated to be in multiple locations simultaneously.
What that fact implies about the nature of reality on the observable level (cats, for example, as opposed to electrons) is one of the stickiest areas of quantum physics. We're in one universe if we see that the cat is alive at the end of the test period, and we are in another one if we see that the cat is dead. But not everyone agrees with this conclusion. Einstein said, "I refuse to believe that God plays dice with the Universe." Schrödinger himself is rumored to have said, later in life, that he wished he had never met that cat.
Today, however, if you ask physicists about the existence of alternate universes, most of them will agree that it is a definite possibility. The cover of the February 17 edition of Time features a story about "the infinity machine," a supercooled computer which according to one theory of quantum physics, is "the first techniology that allows useful tasks to be performed in collaboration between parallel universes." Time states that research with these computers, each one costing $10,000,000, is "backed by Jefff Bezos, NASA, and the CIA," and is proceeding rapidly.