They could be von Neumann probes having travelled for a long time. Even without FTL travel, von Neumann probes could colonize an entire galaxy in reasonable time (a few million years). Even just a single successful civilization somewhere in the milky way or even a neighboring galaxy could've accomplished that
Especially since at the point any civilization starts to develop technology, it happens really exponentially. Even just 1 million years, which is nothing compared to geological time scales, is an absolutely insanely long amount of time to develop technology after the initial spark.
Our 300,000 year old species didn't even settle down until a mere 12,000 years ago, we didn't really develop proper government systems until 2-3,000 years ago and learnt flight only ~120 years ago.
Within the last 120 years we went to the Moon (!), developed the integrated microchip, the Internet, AR/VR and built deep space solar communications infrastructure, got smartphones, MRTs... you name it.
Even among all of these marvels, we still poop though.
but if you want a list of hypothetical alternate origins, and don’t have access to the wealth of programming generated by Prometheus and made available on Discovery Networks, thanks to some of the minds of our time, this light summary might help…
* center of the earth is actually hollow, “aliens” are just inner earthlings
* one of the other planets or otherwise within the solar system isn’t so dead after all
* it’s just us - from the future
* it’s just us - from the past
* no space travel, no time travel, but parallel earths or dimensions or something
* they have always been here, we just are too dumb to see them
