User: What does this mean: "Steady-state theory"?
Assistant: In cosmology, the steady-state model, or steady state theory, is an alternative to the Big Bang theory of the evolution of the universe. In the steady-state model, the density of matter in an expanding universe stays the same because matter is always being made. This fits with the perfect cosmological principle, which says that the universe we can see is pretty much the same everywhere and at any time. 

Problems with the steady-state model began to emerge in the 1950s and 60s, when observations began to support the idea that the universe was in fact changing: bright radio sources (quasars and radio galaxies) were found only at large distances (therefore could have existed only in the distant past due to the effects of the speed of light on astronomy), not in closer galaxies. 

The discovery of the cosmic microwave background radiation in 1964, which the Big Bang theory had predicted, provided the majority of cosmologists with a conclusive challenge to the steady-state model.