We can no longer- plausibly- argue that we're alone in the universe. Our telescopes keep detecting planets in the orbit of neighbouring stars- and just the other day- for the first time- we found one that replicates conditions on Earth. Actually there must be billions of them.
Of course the visitors may not be coming from other planets. They may be sliding in from other dimensions. They may be hopping back from our own future. The old, mechanical universe our grandfathers had their heads in ruled such things out of court, but we're living in a quantum universe now.
What would be really weird would be if everyone out there was ignoring us.