Short answers, pulled from the story.
The multiverse is the hypothetical set of all universes, presumed to comprise everything that exists: all space, time, matter, energy, information, and physical laws. Critics argue that because it cannot be empirically falsified, it functions more as a philosophical notion than a scientific hypothesis.
The Ancient Greek Atomists, beginning with Leucippus and Democritus in the fifth century BCE, are the first figures to whom historians can definitively attribute the concept of innumerable worlds. Epicurus (341-270 BCE) and the Roman Epicurean Lucretius in the first century BCE continued the tradition.
Tegmark's classification runs from Level I (an infinite universe containing identical Hubble volumes), Level II (bubble universes with different physical constants from eternal inflation), Level III (the many-worlds interpretation of quantum mechanics), to Level IV (the ultimate ensemble of all mathematically possible universes).
No statistically significant evidence has been confirmed. Around 2010, analysis of WMAP and Planck satellite data found no sign of bubble universe collisions. In 2015, Dr. Ranga-Ram Chary identified an anomalous signal in the cosmic radiation spectrum 4,500 times brighter than expected, which could indicate matter from a parallel universe, but there is a 30% chance the signal is noise.
Cosmologist George Ellis argued in August 2011 that the multiverse lies so far beyond the cosmological horizon that evidence is unlikely ever to be found, placing it outside testable science. Stoeger, Ellis, and Kircher note that universes in a true multiverse are causally disconnected from one another, which puts the concept beyond scientific support.
The weak anthropic principle holds that conscious observers can only exist in a universe whose physical laws permit life. If a vast number of universes exist with varying constants, observers would naturally find themselves in one of the rare life-permitting universes, explaining apparent fine-tuning without invoking intelligent design.