Big Bang theory represents the most important theory in astronomy, which explains the beginning of the universe.
This theory states that the expansion of the ball of compacted matter, having infinite density, called a singularity, led to the birth of universe.
Singularities are accepted to be at the nucleus of black holes, which are regions of strong gravitational pressure, this pressure creating compression of finite matter till it has infinite density.
Scientists believe that there still exists heat produced by Big Bang (CMB), since the universe continues to expand further. The current expansion of the universe theory is supported by observations made over galaxies, which are moving away from our galaxy, at a speed that is directly proportional to the distance between them.
The basis of the Big Bang theory, then, is the "echo" of the big bang that remains as a heat signature that is called the cosmic microwave background (CMB) and that is found to be all throughout the universe as a background radiation.
There also exists theories that claim that the beginning of existence of time and space coincides with the Big Bang. The origins of our universe are explained by other theories, also, Big Bang theory being one of them.