The idea that there could be more than one Universe has been discussed since the times of the Ancient Greek philosophers in the sixth century BCE.

Multiverse theories provide a neat sidestep away from many of the unexplained observations found in the Universe we know, For example explaining why the physical constantsplugin-autotooltip__plain plugin-autotooltip_bigPhysical constants

Constants variability :

Many physics calculations rely on the assumption that the ‘physical constants’ e.g. light-speed, gravity, etc etc are, in fact, constant. Extremely accurate experimental procedures strongly suggest that they (mostly) are. But the experiments can only be carried out on a ‘local’ scale, and in a very short timeframe (cosmologically speaking). Thus the possibility exists that some constants may vary at extreme distances and/or timescales. If so, current … have the values which they do. If there are an infiniteplugin-autotooltip__plain plugin-autotooltip_bigInfinity

unknowable

If a mathematician wants to explore infinity, there are many options - for example by calculating π , or the square root of 2, or dividing any number by 0.

For philosophers, the concept of infinity is fraught with enigmas :

* Are infinities created over time different from instant infinities? number of universes, then there will surely be at least one like the one we inhabit - with the same 'laws of physics', the same quantum-physics phenomena, and the same values for the constants etc. etc..

Although many groups of cosmologists and particle physicists take the core ideas very seriously, otherse point out that, since we don't have access to any other universes, the theories can never be proved or disproved (hence the 'Unfalsifiable' tag above)

As Wikipedia puts it :

Some physicists argue that the multiverse is a philosophical notion rather than a scientific hypothesis, as it cannot be empirically falsified. In recent years, there have been proponents and skeptics of multiverse theories within the physics community. Although some scientists have analyzed data in search of evidence for other universes, no statistically significant evidence has been found. Critics argue that the multiverse concept lacks testability and falsifiability, which are essential for scientific inquiry, and that it raises unresolved metaphysical issues."

Source Wikipedia

A considerable number of very high-profile cosmologists have supported the ideas, whilst a roughly equal number completely dismiss them (ref.)

One certainty however, is that to date, no practical experiments of any kind have provided hard evidence of the existence of any other universe.