NASA WebGL of the Universe (opens in new tab)

Several of the methods used for finding exoplanets are biased so that they're more likely to find exoplanets that are both really big (Jupiter-size or bigger) and really close to their star (much closer than Earth)

As methods of detecting exoplanets get better, I think they're starting to find more systems with multiple planets so the average is probably more than 1

Not an astronomer but if I'm not mistaken, we find exoplanets based on the shadows from their respective stars, we can usually tell if a star has multiple planets orbiting based on the amount of light we see, it's just that it's strangely more common to only have 1 planet orbiting each star, instead of the many we have.

Yeah, that's exactly why they're focusing on the Milky Way and not the whole universe, and thus the title is a little bit off.