Up until the 1920s, there was a predominant idea in astrophysics about the universe. This was that it was static and everlasting. Most observations fit in with this idea somewhat well, until Edwin Hubble came along. In 1929, Edwin Hubble made a monumental observation that changed the course of astronomy. He discovered that galaxies far away from us have a red shift. This means that they are moving away from us and thus, must have been closer together at some point; and so, ultimately they must have existed in one point in space, a singularity. During this moment, all laws of science would break down and not exist, so events that occurred before would have no effect on the present outcome, the cosmos that exists today.
Many, however, were still skeptical about this idea of an expanding universe. These people argued that if the big bang was such a massive explosion, it must have therefore produced a tremendous amount of heat. If that were so, then remnants of this heat should still be detectable in our present cosmos. This turned out to be true. Through a series of measurements, scientists have discovered that, at present, the temperature of the universe is approximately 3 degrees Kelvin. This is a stunning discovery that has made the Big Bang Theory one that most physicists today now believe.