During the hypothetical "black hole era" of the universe, which is a possible scenario based on current astrophysical understanding, black holes would indeed be a prominent feature. However, it is important to note that our understanding of the far future of the universe is still speculative and subject to ongoing scientific research and exploration. The concept of a "black hole era" is based on extrapolations from current knowledge and theories.
In this scenario, as the universe continues to expand, galaxies would gradually disperse, stars would exhaust their fuel, and ultimately, stellar remnants would evolve into black holes. Over an immensely long timescale, these black holes would dominate the universe.
However, it is crucial to recognize that our current understanding of physics is incomplete, particularly when it comes to reconciling general relativity (describing gravity on large scales) with quantum mechanics (governing subatomic phenomena). The behavior of matter and energy under extreme conditions, such as those inside black holes or during the black hole era, is still an area of active research.
Therefore, while it is plausible that black holes would play a significant role in the distant future, we cannot definitively state that only black holes would exist during the hypothetical black hole era. Further scientific advancements and discoveries are necessary to provide a more accurate picture of the fate of the universe.