Chances are ETs have transformed into a physical form that is undistinguishable from, say, planets, energy, even stars. After all, the ultimate level of technology design and sophistication is when it is not distinguishable from its surroundings.

Agreed - we do not even possess the instruments to define the nature of dark matter, and it is only by inference that we can suppose it exists.

It is amazing that Fermi, Einstein, et al were able to predict what they did without readily available modern computing hardware and search engines.

Some things are out of the realm of our perception.