It's part of the idea 'there is' like in your example There's a strange cat in the garden, which we use to state what does or not exist. God ordered the existence of the World: "And there was light". So, metaphysically speaking, you cannot divide there from is.
It makes a bit of sense to me personally to consider it an adverb, but not a lot. I'm struggling to see how it can be a preposition at all - can you show how?
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