The causality of events post the Big Bang is clear. But if we consistently "move" backwards, at the most basic level of physical reality's existence, it should culminate where matter, as the "prime cause of the world", should not possess a reason for its existence and a cause for its initial motion. It simply exists just without a cause and possesses the opportunity to spontaneously evolve.
Reflecting on the finitude and infinity of the world, if we assume that the world is a Multiverse and it is not infinite, then what can be imagined beyond its limits? Absolute emptiness, or is there a logical model where the emptiness disappears?
For a world like ours to be possible, it perhaps must have a beginning rather than being beginningless. And a beginning implies the existence of some substance, which, in turn, suggests its eternal existence. Thus, our reality is such that something has always existed and will always exist. If this is logical, then how can one describe the possible characteristics of this substance? One option is that it's an eternally immobile matter, yet unstable, spontaneously and without cause set into motion, or more clearly — space, capable of expanding based on its internal energy and giving rise to Universes similar to ours, a space that, along with its components, we now refer to as the Multiverse.