Which SciFi ttrpg has the largest player base?

[Glorthindel]

As far as Warhammer 40k goes, I have noticed from a long time in the community, that there is a notable UK and EU skew to its popularity, with it being notably less popular in the US. It obviously makes sense that it has strong UK popularity (my entire friend circle lives within 20 miles of Games Workshops HQ), but I have always wondered why it was so popular in the EU, considering how much of the warhammer world is parodying european nations.

I notice no one has mentioned Alternity. Granted, its first edition has been dead as long as TSR, but I know it had a kickstarter new edition in the last few years (as my brother got it), but never seen anyone playing it.