even granting your assumptions, we ARE alone. Actually, it is quite likely that EVERY sentient life form that exists is alone. The distances between them will almost certainly be too large even for discovery of each other. The distances will be way too large for contact, way way too large for conversation, way way way too large for collaboration, and WAY WAY WAY WAY too large for visits. (That's assuming nobody discovers totally unanticipated physics that allows warp drive or worm holes or some such. It makes for fun sci fi, but don't get your hopes up.) PLUS, who's to say that every sentient life form will develop science and have an interest in other worlds? PLUS, societies might self-destruct, or use up all their fossil fuels (or never have any), or might have started a million years after us, or a million before (in which case we'd have nothing in common with them).
Of course, as a Christian, I believe we are NOT alone. But that's a whole different discussion!