Simply put: we don't know yet. But it seems a bit too confusing to answer "I don't know" directly, first explain "why don't we know", and then try to use quantum mechanics to explain how the Big Bang actually happened.
Scientists can only manage what happened after the birth of the universe, and "what really happened at the moment of the Big Bang" is not in the hands of scientists.
How to understand the above sentence?
Planck time, human cognition can only be from the Big Bang after one Planck time. Because Planck time is the smallest unit of time that makes sense, any unit smaller than Planck time makes no sense.
So if you were to ask "what was the state of the Universe before the Big Bang was a Planck time," that question is meaningless to humanity, at least not at the moment.
A Planck time is very short, as for 10^-43 seconds. That is to say, scientists can only describe the universe 10^-43 seconds after the Big Bang, and the previous universe is meaningless to us.
The singularity of the Big Bang was just a mathematical concept, the singularity is the zero point, and everything starts from scratch. Although planck time is very short, it is always greater than zero.
For the recognition of singularities, all the laws of physics of mankind are currently invalid.
But weird quantum mechanics seems to be taking our understanding of the Big Bang a step further.
According to the interpretation of quantum mechanics, in the process of matter collapsing inward under the action of gravity, it is possible to "bounce" and expand into other dimensions, the so-called high dimensions.
As for how the higher dimensions exist, it is not something we can imagine.
For example, the magical central singularity of a black hole is like this, the black hole singularity can collapse infinitely on space-time, so that space-time no longer exists at the singularity, so that all the laws of nature we know are disappearing at the singularity.
But according to the interpretation of quantum mechanics, quantum bubbles (quantum ups and downs) may appear at any time, and quantum bubbles from time-space continue to play out, and space-time energy will appear in our universe.
From this point of view, the energy is still conserved, the difference is that the energy of the two different dimensions of the quantum is exchanged to achieve equilibrium.
What is a quantum bubble (quantum undulation)?
A quantum bubble is a rise and fall of energy called "vacuum zero energy." The quantum bubble is randomly staged, then disappears in an instant, and is hidden in a vacuum.
The way it appears is the derivation and annihilation of pairs of virtual particles, and the time of derivation is very short, almost instantaneously disappearing. According to the interpretation of quantum mechanical uncertainty, there will also be virtual particles that have not been annihilated by chance, so singularities may appear.
This quantum bubble is considered by scientists to be a state before the Big Bang. The singularity expands from other dimensions of space-time into our universe, and then space-time is formed.
For the ultimate future of the universe, there is a view that the distant future universe will stop expanding, then begin to collapse inwards, and eventually form singularities again and begin a new cycle!