I know the Avatamsaka Sutra teaches about a multiverse flower garland, which represents all of reality, is made up of infinite universes which all reflect one another.
Is this the only scripture that talks about multiverses?
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The Avatamsaka Sutra speaks of "worlds" or "realms", not of universes, and certainly not about the scientific theory of the multiverse:
In all atoms of all lands Buddha enters, each and every one, Producing miracle displays for sentient beings: Such is the way of Vairocana.... The techniques of the Buddhas are inconceivable, All appearing in accord with beings’ minds.... In each atom the Buddhas of all times Appear, according to inclinations; While their essential nature neither comes nor goes, By their vow power they pervade the worlds.
The characters written in the text for this are usually 境界 (Jìngjiè), where 境 means "territory" and 界 means "world", usually rendered in English as "realm".
For "universe", the characters 宇宙 (Yǔzhòu) would be used. These do not appear anywhere in the Chinese text.