I have to step in, people are using terms too interchangeably. Nerds do very well academically (I saw a talk show host talking about too many people claiming to be nerds, "No, you're not"). Geeks are knowledgeable enough to know a lot about the topic they're into, e.g. anime. Dorks are socially inept, generally into stuff that isn't considered cool by many, and may or may not be smart. Or a person can do specific things that are dorky, as geeks and nerds may do, or anyone else. Nerd and geek is mainly a whole persona. Nerds and geeks may share some consumer interests, but geeks more make their identity. So I would say the closest to Otaku is geek. Geeks can be more of the hip type, or more dorky types. It sounds like Otaku leans more towards the latter. I would just say it as,
In Japan, table tennis is considered to be a dorky sport for geeks.
Or maybe just,
... a sport for geeks.
If you want to sound nicer about it. But 'dorky' can be taken in jest also, like, "Omg, you're such a dork" when someone does something silly, like getting all up in arms about people mixing up these terms.
Here is a video that compares and contrasts nerds and geeks.
Here are definitions,
Nerd,
a person devoted to intellectual, academic, or technical pursuits or interests
Geek
1
: a person often of an intellectual bent who is disliked
2
: an enthusiast or expert especially in a technological field or activity
Dork
an odd, socially awkward, unstylish person
And to agree with others, no, that is not the right usage of 'dark', as neither is 'gloomy'.