(Bloomberg) -- If you've been paying attention to auction coverage recently, the outlook seems grim: The value of artworks on offer has plummeted, volume and totals are down significantly, and various news organizations (including this one) have sounded ominous warnings of an imminent market decline.

But all of this is drawn from a tiny sliver of the art market. Bloomberg's auction coverage, for instance, is largely confined to Christie's, Sotheby's, and Phillips. Not only does that exclude tens of thousands of smaller auction houses around the globe, it also omits the majority of art buyers. Only a few thousand people on the planet can afford to spend a million dollars on a painting, but many millions have paintings in their houses. (It's also worth pointing out that even millionaires often fly under the radar by buying art at galleries and through private sales.) We have no idea, in other words, how strong or weak the market really is.

That's beginning to change, though, thanks to an online auction service that many people—even art-world insiders—have never heard of. Invaluable, which was founded in 1989 as the database company Artfact and then repurposed into its current form in 2009, is in essence an online portal: Collectors search for, say, Andy Warhol prints, and the site will display upcoming lots from more than 4,000 brick-and-mortar and online auction houses, most of which fall within the low to middle end (lots below $100,000) of the market. Users can then bid on these lots through Invaluable's site; the company makes its money by charging the auction house a transaction fee (about 5%) when an online bidder buys an object.

Because it represents so many auction houses, Invaluable has compiled a vast database of past art sales, perhaps the largest of its kind in the world. (Other databases, such as Artnet, have robust data in contemporary and modern art categories but are less comprehensive in others.)

Private collectors, companies, and institutions can pay a fee to access Invaluable's database; the company says that, along with some of the world's biggest dealers and galleries, the Museum of Modern Art in New York, the Louvre in Paris, and even the IRS subscribe to its services. "Auction houses, too," Invaluable's chief executive officer, Rob Weisberg, said in an interview. "We make it available to them so that they can be better appraisers."

Until now, however, Invaluable hadn't released any aggregate data, which meant it was sitting on a lockbox of analytics and market insight. After some prodding, the company has agreed to pry it open, but only just.

In 2014, according to Invaluable, the 1,400 auction houses it represents that sell fine art (a category that includes paintings, sculpture, prints, drawings, photography, and posters) had a total of $607.9 million dollars in sales. Last year that number grew to $681.8 million, or a 12% increase. Drilling down a more esoteric category, we can look at the year-over-year growth for Asian art and antiquities: In 2014 the total was $237.3 million; the next year sales increased 17%, to $276.6 million.

Again, this is a limited snapshot. Invaluable declined to release the number of objects in its totals, which means, conceivably, that the 12% growth could have been derived from a single, multimillion-dollar sale. The company also declined to release the sales data from previous years, which means the 2014-15 year-over-year growth could be an anomaly.

Still, the data is useful; even though these numbers pale in comparison with the billion-dollar totals at Christie's and Sotheby's, this is some of the first information to come out of heretofore overlooked auction houses. (Now is a good time to mention that other market reports already exist, like the one released by the European Fine Art Foundation, though its research is by no means exhaustive.)

Invaluable's totals are helpful enough, said Marion Maneker, editor of Art Market Monitor, to make some (careful and caveat-riddled) extrapolations. "We're seeing a thriving market for lower-placed works," he said. "Participation is going up, and dollar amounts are going down."

Put another way, Invaluable's data appear to demonstrate that there's been a trickle-down effect: The massive, headline-drawing sales seem to have convinced buyers that art is an investment worth making.

"We saw a run from 2010 to 2015 that was primarily in the so-called masterpiece market," said Maneker, referring to works roughly above $10 million. "That's running out of steam now, but it's given enough time for the middle market to build itself up with plenty of participants who have plenty of confidence."

This could be heartening news to many dealers and auction houses. The market has profound growth potential if the thousands of people who spend $1 million are supplanted by millions who spend $1,000. At the moment, it appears that's exactly what's happening.

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