Final answer:
Theories about the origin of the universe revolve around the Big Bang theory, which is supported by evidence such as cosmic microwave background radiation and the expanding nature of the universe. The early universe was hot and dense, and much of its mass and energy is still not well understood.
Step-by-step explanation:
Theories about the origin of the universe typically revolve around the Big Bang theory, which proposes that the universe began with a single event from a single point. This theory is supported by various pieces of evidence, such as the cosmic microwave background radiation and the isotropic and expanding nature of the universe. According to this theory, the early universe was hot and dense, and the vast portion of the mass and energy of the universe is still not well understood.