The Steady State Theory was developped in 1949 as an alternative to the Big Bang theory. According to the Steady State theory, although the universe was expanding, it nevertheless did not change its onlook over time, because new matter was formed to keep the density equal. Because only very little matter needs to be formed, such forming of matter had not been proven directly.
Problems with the Steady State Theory began to emerge in the late 1960s, when evidence started to show that the universe was in fact changing: quasars and radio galaxies were only found at large distances (and thus, because of the finiteness of the speed of light, in the past), not in more nearby galaxies. The definite blow came with the discovery of the cosmic background radiation in 1965 and its explanation. Nowadays, few if any astronomers do not believe in the Big Bang theory.