It is commonly known among serious scientists
and particularly among astronomists that there was a Big Bang
at the beginning of the universe. One evidence for it is the
fact that the universe is expanding with a steady (or decreasing,
or increasing) rate.
This has been supported by the data collected by
the Hubble Telescope, see Universe development,
Expansion. Edwin Hubble was the very founder of the theory
of the expansion of the universe (see "The Birth of Time" by John
Gribbin, Weidenfeld & Nicholson, London 1999).

Before this Big Bang, all agree, matter must have
been concentrated in a hugely dense and solid mass, the size
of which nobody knows. Scientists conjecture by estimating density
and size of the known universe. They interpret the difference
between electromagnetic waves, like the colour of light, that
can be measured in distant galaxies. This is the redshift. There
must have been a big explosion in the beginning, as shown above.