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.
