The initial singularity was the gravitational singularity of infinite density thought to have contained all of the mass and space-time of the Universe before quantum fluctuations caused it to rapidly expand in the Big Bang and subsequent inflation, creating the present-day Universe. The initial singularity is part of the Planck epoch, the earliest period of time in the history of the universe.
Initial singularity Wikipedia
General relativity is used to predict that at the beginning of the Universe, a body containing all mass, energy, and spacetime in the Universe would be compressed to an infinitely dense point. The use of only general relativity to predict what happened in the beginnings of the Universe has been heavily criticized, as quantum mechanics becomes a significant factor in the high-energy environment of the earliest Universe, and general relativity on its own fails to make accurate predictions. In response to the inaccuracy of considering only general relativity, as in the traditional model of the Big Bang, alternative theoretical formulations for the beginning of the Universe have been proposed, including a string theory-based model in which two branes, enormous membranes much larger than the Universe, collided, creating mass and energy.
It is impossible to see the singularity or the actual Big Bang itself, as time and space did not exist inside the singularity and, therefore, there would be no way to transmit any radiation from before the Big Bang to the present day. However, evidence for the existence of an initial singularity, and the Big Bang theory itself, comes in the form of the cosmic microwave background and the continued expansion of the Universe.
Various new models of what preceded and caused the Big Bang have been proposed as a result of the problems created by quantum mechanics. One model, using loop quantum gravity, aims to explain the beginnings of the Universe through a series of Big Bounces, in which quantum fluctuations cause the Universe to expand. This formulation also predicts a cyclic model of universes, with a new universe being created after an old one is destroyed, each with different physical constants. Another formulation, based on M-theory and observations of the cosmic microwave background, states that the Universe is but one of many in a multiverse, and has budded off from another universe as a result of quantum fluctuations, as opposed to our Universe being all that exists.