M87, discovered by the French astronomer Charles Messier in 1781, is the strongest radio-emitting object in the constellation Virgo. M87 has a diameter of about 120,000 lightyears, and contains on the order of 2.7 trillion stars. At radio wavelengths, M87 looks like this:

Speculation:

If individual stars (perhaps civilizations) are the source of radio emissions, how many civilizations are we talking about? Maybe 1 trillion x 10 billion people each? That's 10^21 alien intelligences. This assumes that they all use their own stars to generate microwaves. These microwaves can be converted to electrical power with 80 percent efficiency using a rectenna. Solar concentrating mirrors and the Rankine cycle can only achieve perhaps 20 percent efficiency. So they would immediately realize a 4-fold increase in power production for the same amount of infrastructure (at our level of technology). Alternatively they could reduce the mass of their orbiting installations by 4-fold and generate the same amount of power.

We already know that interstellar communication is a non-possibility with any conceivable technology, due to the time lag. However, radio waves or microwaves could be evidence of power production, conversion, and transmission. There would be no 'encoding' indicating information is being transmitted, just pure energy.

I wonder if a spacecraft could be built to siphon relativistic electrons from the Sun's corona and use them to generate power? I wonder if it would be better than photovoltaics?