The most popular theory for the origin of the Universe is the Big Bang Theory, according to which the Universe expanded to its present enormous volume from an initial miniscule starting volume. This expansion has taken place over the past 10 or so billion years.
Two observations, both made during the 20th Century have profoundly shaped the way in which we think about our Universe and have lead to the Big Bang Theory for the origin of the Universe. First was the discovery by Edwin Hubble that our Universe is expanding. Second was the discovery made in 1965 by Arno Penzias and Robert Wilson that our part of the Universe is filled with microwave radiation. This radiation has become known as the Cosmic Microwave Background.