The idea that we can't detect alien civilisations through radio emissions, so therefore those civilisations don't exist, is just nonsensical. Look at humans, we went from plain analogue radio to digitally encrypted low power transmissions in around 100 years, which is the blink of an eye in terms of the lifespan of a planet, or indeed any civilisations on it. Digital radio just looks like noise unless you know it is encrypted and know how to decrypt it. Even if you could work that out, our spread-spectrum transmissions (https://en.wikipedia.org/wiki/Spread_spectrum
) would make it near impossible for another civilisation to decrypt our transmissions because they would need to understand how the transmissions are being spread.
To anyone not privy to the details of our radio technology, our transmissions just look like noise. Why would another civilisation's transmissions be any different?
Besides, much of our comms is now not even radio, but rather fibre optic transmission which is totally enclosed and undetectable. All this in 100 years. To think that we are the only civilisation out there just because we can't hear any others is ludicrous.