These 15 startups didn't exist 5 years ago - now they're worth billions

Every week this year, another startup has reached a billion-dollar valuation. (Whether these private market valuations actually hold water is another question.)

We've decided to rack up a bunch of these billion-dollar startups - Cowboy Ventures' Aileen Lee first referred to them as "unicorns" - that didn't exist five years ago, and that are now valued at $1 billion or more.

For the purposes of this list, we zeroed in on US-based companies that were founded in 2011 or later (since we're nearing the very end of 2015), and that are private tech companies.

We then ranked them from least to most valuable.