Bondi, Gold and Fred Hoyle developed this theory. According to this theory, the number of galaxies in the observable universe is constant and new galaxies are continuously being created out of empty space, which fill up the gaps caused by those galaxies. Which have crossed the boundary of the observable universe.
As a result of it, the overall size of mass of the observable universe remains constant. Thus, a Steady State of the universe is not disturbed at all.