the universe is expanding (which is why it isn't so hot or dense
anymore)
Note that the basic Big Bang Model does NOT say anything about the following
questions:
will the universe collapse again, or expand forever?
is space curved or flat?
how old is the universe?
what is the matter density in the universe?
what about dark matter?
is there some mysterious "repulsive" force on large scales?
how did galaxies form?
Some of these questions all depend upon the values of certain parameters in
the model, which we may derive from observations. Others have nothing to do with
the Big Bang itself.
Our understanding of the laws of nature permit us to track the physical state
of the universe back to a certain point, when the density and temperature were
REALLY high. Beyond that point, we don't know exactly how matter and radiation
behave. Let's call that moment the starting point. It doesn't mean that
the universe "began" at that time, it just means that we don't know what
happened before that point.
