[runawaybottle]

What could create a natural radio signal in space?

Edit: I should have done some basic googling, turns out a few things:

https://en.m.wikipedia.org/wiki/Astronomical_radio_source

[sizzzzlerz]

The real question you should be asking is what doesn't. Radio signals are generated in many ways at frequencies up and down the spectrum.

[magduf]

Magnetars https://en.wikipedia.org/wiki/Magnetar