These 13 startups have raised millions - but no one knows what they do

Before a startup launches, it goes through its stealth phase. Often this is a pretty short time before it hits market, but some companies will take years and raise millions along the way before the public learns what they're doing.

CB Insights, a venture capital data firm, assembled a list of the top-funded stealth companies that haven't opened to the public yet. We excluded one name, Jut, because it has already opened up its beta to the public.

To be considered among the most well-funded, the company had to actively raise money in the last two years and raise at least $10 million.

Here are the companies that have raked in millions even though the public has no idea what they do: