It could have no physical explanation at all. Or there could be endlessly repeating cycles, each a universe in its own right, with the initial quantum state of each universe explained by some feature of the universe before

The Big Bang theory states that the universe originated from a singularity, which was an extremely hot and dense point. The singularity is not the same as nothing. The Big Bang theory also states that the singularity didn’t occur in an already existing space. Instead, it started the expansion and cooling of space

The Big Bang theory also assumes that space was filled with an unstable form of energy before the Big Bang. At some point, this energy was transformed into fundamental particles, which created all the matter we see today. This point is known as the Big Bang. 

The law of conservation of energy states that energy can’t be created or destroyed. It can only be converted from one form of energy to another

The Big Bang theory says that the universe came into being from a single, unimaginably hot and dense point (aka, a singularity) more than 13 billion years ago. It didn’t occur in an already existing space. Rather, it initiated the expansion—and cooling—of space itself.

The Big Bang theory also states that: 

• Before the Big Bang, space and time didn’t exist. 
• The singularity contained all the energy, matter, space, and time of the universe. 
• The explosion that started the expansion was due to infinite heat and gravity crunches. 

The Big Bang theory doesn’t explain what happened before the Big Bang

The cause of the Big Bang is considered a philosophical question. Some ideas about what caused the Big Bang include: 

• Multiverse theory: The universe is part of a collection of universes. 
• Cosmological argument: The universe was created by an infinitely powerful mind, also known as God. 

Two major scientific discoveries support the Big Bang theory: 

• Hubble’s discovery: In the 1920s, Hubble discovered a relationship between a galaxy’s distance from Earth and its speed. 
• Cosmic microwave background radiation: Discovered in the 1960s. 

Other evidence for the Big Bang includes: 

• Redshift of galaxies 
• Microwave background 
• Mixture of elements 

The Cosmic Microwave Background (CMB) is considered a key piece of evidence for the Big Bang. It’s the cooled remnant of the first light that could travel freely throughout the universe. Scientists consider it an echo or “shockwave” of the Big Bang.

The universe began, scientists believe, with every speck of its energy jammed into a very tiny point. This extremely dense point exploded with unimaginable force, creating matter and propelling it outward to make the billions of galaxies of our vast universe.

Here’s some more information about the Big Bang theory: 

• Hubble’s law Hubble’s law states that the more distant a galaxy is, the faster it’s moving away from Earth. 
• Redshift Light from galaxies that are moving away from Earth is shifted to longer wavelengths, making it appear redder. 
• Cosmic microwave background The cosmic microwave background (CMB) is the remnant of the first light that could travel freely throughout the universe.

The Big Bang theory, developed in 1927 is considered the most credible scientific explanation of how the Universe was created. It suggests that through a process of expansion and explosion hydrogen gas was created which led to the formation of stars, and their death (supernova) led to the creation of life.

The Big Bang theory predicts that the early universe was hot and dense, and that this would leave behind relic thermal radiation. This radiation would be redshifted into the microwave domain. The Big Bang theory also predicts that the CMB would be detectable as a subtle “wallpaper” coming from everywhere behind all galaxies, quasars, and galaxy clusters. 

The CMB spectrum has become the most precisely measured black body spectrum in nature. The Big Bang theory also predicts the anisotropies in the cosmic microwave background. 

The CMB is being increasingly redshifted as the Universe expands. After another few trillion years, the current cosmic microwave background will have redshifted into insignificance and will no longer be detectable.

This theory predicted that the ‘shockwave’ of that primeval explosion would be still detectable as a subtle ‘wallpaper’ coming from everywhere behind all galaxies, quasars and galaxy clusters. Today, the Big Bang model is still the only model that is able to convincingly explain the existence of the CMB(full article source google)

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