- We expect advanced life to substantially disturb the places it colonizes. Whenever natural systems are not ideally structured to support colonists, we expect changes to be made. And unless ideal structures always either closely mimic natural appearances or are effectively invisible, we expect advanced life to make visible changes.
For example, it only takes a small amount of nuclear waste dropped into to visibly change its spectra [Whitmire & Wright 80.] And a civilization might convert enough of a star's asteroids into orbiting solar-energy collectors to collect a substantial fraction of this star's output, thereby substantially changing the star's spectral, temporal, and spatial appearances. Even more advanced colonists may disassemble stars [Criswell 85] or enclose them in Dyson spheres well within a million years of arrival. Galaxies may even be restructured wholesale [Dyson 66].
If such advanced life had substantially colonized our planet, we would know it by now. We would also know it if they had restructured most of our solar system's asteroid belt (though much smaller colonies could be hard to detect [Papagiannis 78]). And they certainly haven't disassembled Jupiter or our sun. We should even know it if they had aggressively colonized most of the nearby stars, but left us as a "nature preserve".
Our planet and solar system, however, don't look substantially colonized by advanced competitive life from the stars, and neither does anything else we see. To the contrary, we have had great success at explaining the behavior of our planet and solar system, nearby stars, our galaxy, and even other galaxies, via simple "dead" physical processes, rather than the complex purposeful processes of advanced life. Given how similar our galaxy looks to nearby galaxies, it would even be hard to see how our whole galaxy could be a "nature preserve" among substantially-restructured galaxies.
These considerations strongly suggest that no civilization in our past universe has reached such an "explosive" point, to become the source of a light speed expansion of thorough colonization.