People say we’d see signs of alien civilizations capturing the energies of stars and such - this would imply large objects. But, I think it makes more sense to go small if you wish to colonize a galaxy. You’d want to become a nanobot and have a way to replicate quickly and often. This would require less energy and you could go further just by riding object between star systems.
Discussion
Like bacteria?
Bacteria would be more like biological payload to seed worlds. Nanobots would have to be non-biological swarm intelligence. It feels like we’re not that far from this reality (intelligence and goal seeking part). Need to address the replication part which I hear is an area of ongoing experiments currently.