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How does modal realism treat possible worlds?

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In the context of philosophy and cosmology, modal realism is the view that all possible worlds exist with equal reality. Rather than treating possible worlds as abstract concepts or mere theoretical tools, modal realism asserts that they are concrete, physical entities.12

As developed by the American philosopher David Lewis, modal realism treats possible worlds as a vast plurality of systems that are each spatiotemporally and causally closed. Each world is disjoint from all others and contains its own unique collection of concrete particulars, including all their properties and relations.2

Under this framework, possible-world semantics are considered a literally correct account of modal propositions. Specifically:2
* A proposition is necessarily true if it is true in all possible worlds.2
* A proposition is possibly true if it is true in at least one possible world.2

Consequently, modal realism posits that if any statements about necessity or possibility are true, then these other worlds must exist as real, concrete entities in addition to our own.2