I've met people and have found friends that can relate/are into the stuff i'm into, like we have at least one thing in common. I can't really think of anything that I'm exclusively into and only like 0.01 percent of the population visits that subreddit or whatever lol. Well, actually, some message boards/forums I'm still very active on and I can 100% guarantee nobody in my IRL circle knows about them but I post on them every day lol. Like IGN's vestibule for example.
Music stuff:
I used to be diehard metalhead. I would listen to stuff like cannibal corpse and the faceless and stuff. after high school i chilled out a bit and got more into EDM and other genres, but I can still relate and appreciate metal music a lot. I feel like most people think all metal is the same and don't know what progressive or death or blackened or djent or any subgenre.
underground rap/hiphop music too. many people are just completely unaware that there is a whole world of music out there besides what you hear on the spotify top 100. stuff like suicideboys, bones/teamsesh, devilish trio (memphis phonk), etc. It goes way deeper than those artists though, like the stuff you find on SoundCloud with only a hundred plays and a few likes, but it's so well produced and slaps and rappers are actually talented and know how to write.
I also agree with the "mallcore" stuff in vaporwave, but to me that's now a dated sound. reminds me of 2014. I was really into synthwave stuff like Com Truise back then. Nowadays I'm more into the really melodic female fronted hyperpop stuff and house stuff. Stuff like Charli XCX which is more mainstream, but I can dig it.
besides music:
I'm really into halloween stuff, like the props and decor and shit. It's all junk but it makes me feel something special. I love collecting old dusty horror novels and often visit my local thrift shops and used book stores to find some strange obscure stuff. same thing with independent horror movies made with a very small budget, there's just something unique, good or bad, I enjoy it. like fan made friday the 13th films on youtube. same thing with low budget horror video games. I obviously love 80's and 90's creepy stuff. goosebumps. are you afraid of the dark. cheesy, campy but lovable and nostalgic. I have so much nostalgia for random 2000's stuff too, like Harry Potter and Chamber of Secrets on PS2 but I feel like nobody talks about that game anymore besides me haha. There's probably a ton of other small things like that which keep me up at night.