There are some very plausible hypotheses in modern Cosmology that use
well-known and long-proven Physics and Math to explain the evolution of
the universe, and account very nicely for the items and structures that
we see all around us in every direction.
The hypotheses have not been demonstrated, because it's really hard to set up an experiment in the lab, and then wait several billion years for the interstellar nebula to condense into a star and planets and life to form on the planets.
So to answer the question, it's true that the processes have not been demonstrated. But it's false that no plausible hypotheses exist that explain the evolution of the modern universe, and that correspond nicely with what we're able to observe of the structure of the universe today.
