The universe has no beginning or end. It remains in a steady state, with new matter continuously created to maintain constant density as the universe expands.
The steady-state theory competed with the Big Bang theory until the 1960s discovery of cosmic microwave background radiation, which strongly supports the Big Bang. Subsequent observations of distant supernovae, cosmic expansion, and abundance of light elements all confirm the Big Bang model. The steady-state theory is now virtually abandoned in cosmology.