In cosmology, the Steady State theory is an alternative to the Big Bang model of the evolution of our universe. In the steady-state theory, the density of matter in the expanding universe remains unchanged due to a continuous creation of matter, thus adhering to the perfect cosmological principle, a principle that asserts that the observable universe is basically the same at any time as well as at any place.
While the steady state model enjoyed some popularity in the mid-20th century, even though the Big Bang theory had more popularity, it is now rejected by the vast majority of cosmologists, astrophysicists and astronomers, as the observational evidence points to a hot Big Bang cosmology with a finite age of the universe, which the Steady State model does not predict. 
Cosmological expansion was originally discovered through observations by Edwin Hubble. Theoretical calculations also showed that the static universe as modeled by Einstein (1917) was unstable and contradicted general relativity. The modern Big Bang theory is one in which the universe has a finite age and has evolved over time through cooling, expansion, and the formation of structures through gravitational collapse.
The steady state theory asserts that although the universe is expanding, it nevertheless does not change its appearance over time (the perfect cosmological principle); the universe has no beginning and no end. This requires that matter be continually created in order to keep the universe's density from decreasing. Influential papers on steady state cosmologies were published by Hermann Bondi, Thomas Gold, and Fred Hoyle in 1948.  
It is now known that Albert Einstein considered a steady-state model of the expanding universe, as indicated in a 1931 manuscript, many years before Hoyle, Bondi and Gold. However, he quickly abandoned the idea. 
Counts of radio sources
Problems with the steady-state theory 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), not in closer galaxies. Whereas the Big Bang theory predicted as much, the Steady State theory predicted that such objects would be found throughout the universe, including close to our own galaxy. By 1961, statistical tests based on radio-source surveys had ruled out the steady state model in the minds of most cosmologists, although some proponents of the steady state insisted that the radio data were suspect.
Cosmic microwave background
For most cosmologists, the definitive refutation of the steady-state theory came with the discovery of the cosmic microwave background radiation in 1964, which was predicted by the Big Bang theory. The steady-state theory explained microwave background radiation as the result of light from ancient stars that has been scattered by galactic dust. However, the cosmic microwave background level is very even in all directions, making it difficult to explain how it could be generated by numerous point sources and the microwave background radiation shows no evidence of characteristics such as polarization that are normally associated with scattering. Furthermore, its spectrum is so close to that of an ideal black body that it could hardly be formed by the superposition of contributions from a multitude of dust clumps at different temperatures as well as at different redshifts. Steven Weinberg wrote in 1972,
- The steady state model does not appear to agree with the observed dL versus z relation or with source counts... In a sense, this disagreement is a credit to the model; alone among all cosmologies, the steady-state model makes such definite predictions that it can be disproved even with the limited observational evidence at our disposal. The steady-state model is so attractive that many of its adherents still retain hope that the evidence against it will eventually disappear as observations improve. However, if the cosmic microwave radiation . . . is really black-body radiation, it will be difficult to doubt that the universe has evolved from a hotter denser early stage.
Since this discovery, the Big Bang theory has been considered to provide the best explanation of the origin of the universe. In most astrophysical publications, the Big Bang is implicitly accepted and is used as the basis of more complete theories.
Quasi-steady state cosmology (QSS) was proposed in 1993 by Fred Hoyle, Geoffrey Burbidge, and Jayant V. Narlikar as a new incarnation of the steady state ideas meant to explain additional features unaccounted for in the initial proposal. The theory suggests pockets of creation occurring over time within the universe, sometimes referred to as minibangs, mini-creation events, or little bangs . After the observation of an accelerating universe, further modifications of the model were made. 
Astrophysicist and cosmologist Ned Wright has pointed out flaws in the theory.  These first comments were soon rebutted by the proponents.  Wright and other mainstream cosmologists reviewing QSS have pointed out new flaws and discrepancies with observations left unexplained by proponents.