I am looking to buy a domain (e.g. bigdata.io) but the .com domain is already registered (bigdata.com). It belongs to a company working in a different field compared to the purpose I have in mind for my domain.
Is there any risk of legal action if I proceed and buy the .io domain?
Not likely. "Big data" are industry terms often used as a phrase. And while there may be a trademark for BigData (I don't know if there is or not), trademarks are not generally defensible if they contain common terms or phrases especially within an industry.
As long as you are not trying to confuse any trademark with any similar look and feel for their site or business model, you should be fine. Otherwise, they could argue that the use of bigdata.io is a violation of their trademark by pointing to the fact that you are trying to dilute their brand by confusing search users.
Run in your own lane and you should be fine.