[Interest Check] I am a fairly new DM, what is the most popular "world" or "theme" for me to set my adventure to?

I'm asking so I can give people what they want and I'd like to know which universe I should read up on.

Well, D&D 5e and Pathfinder are the most popular games, but there is a lot more people looking for games then running them.  It is generally better to focus on what appeals to you, then what is popular.  Good luck and good gaming.

I'm not talking about games I'm talking about "UNIVERSES" "WORLDS" "THEMES" e.g. grey rock, forgotten realms etc.

Greyhawk, Dragon lance, Forgotten Realms, Eberron, Ravenloft, Dark Sun. They add all different elements. Take a look at each setting and figure out which one inspires you more. the more you put into a setting the better the players will feel rewarded. So again its not so much what is more popular. Its more to what gets you motivated to run a good game that will keep players coming back.

Again, as Steely Phil said, so long as you run popular games like D&D and Pathfinder you will get players regardless of the settings. As for 5e I have seen many different worlds being hosted by different GMs and Forgotten Realm is prevalent on those who want to tie them with official adventures, otherwise people go with homebrew worlds. As for themes, in 5e have seen groups being formed in games that featured standard medieval to grim-dark and even D&D in space. The only limiting factor that I have seen thus far is not the world or theme, but rather the time and date. Try lurking on the LFG tool and get a feel of what others are hosting at the moment and how many people are gathering on those time frames.