A single customer made up 19% of Nvidia's revenue last year. UBS thinks it's Microsoft.

• Recent disclosures from Nvidia highlight just how concentrated its customer base is.
• The company said just two customers represented 24% of its revenue in the first quarter.
• UBS thinks the biggest mystery customer — accounting for 19% of revenue last year — is Microsoft.

Nvidia has a few mystery customers that are making up a massive chunk of its revenue base, according to new disclosures.