I believe it is theoretically possible (through to probable) that von Neumann probes have reached our star system. I am not so sure about “aliens”. I am completely out on “other dimensions” (just can’t conceive of this being “possible”).

A third option (although it’s a stretch for sure) is that they share this planet with us, based either in the vast unexplored areas of the oceans or in deep underground caverns. Would explain why they have an interest in shutting down our nukes, since if we blow ourselves up then we would take them with us.