"-dom" is a suffix (a group of letters at the end of a word to form a new word). A word ending in -dom means: an area, a domain, a group of attitudes, a group of people, etc. Examples include: kingdom, officialdom, boredom. English-speakers make up new -dom words all the time. Here, someone means the attitudes and behaviours of nerds. Since nerd ends with a d and dom starts with a d, one d is removed: nerd + dom --> nerdom. The writer knows about nerdom and has now discovered a new, large area of nerdom.

Nerdom: The attitudes and behaviours of a nerd. What is a nerd? A person who is extremely interested in one subject, especially computers, and knows a lot of facts about it.