Final answer:
In a universe without a big bang, there would likely be an absence of cosmic microwave background radiation, which is the afterglow from the early universe's expansion and cooling.
Step-by-step explanation:
In a universe that has existed forever without a beginning and without a big bang, what would likely be absent is the cosmic microwave background radiation (CMB). The CMB is the afterglow of the hot, early universe, formed approximately 380,000 years after the Big Bang when electrons combined with protons to form hydrogen atoms.
As a result, the universe became transparent to light, leading to the dark ages before stars and galaxies began to form. The CMB is a critical piece of evidence supporting the Big Bang theory, as it represents remnants of the early universe's energy and has been measured in all directions in space. In a hypothetical universe without a big bang, the signature CMB would not exist, as it is the result of the universe rapidly expanding and cooling after such an event.