Fingers have been pointed at labor unions, billionaire investor George Soros, billionaire environmentalist Tom Steyer and the Tides Foundation as the supposed liberal counterparts to the Kochs.
But the numbers just don't add up. And these progressive groups tend to operate in the sunshine of public disclosure, unlike the Kochs' semi-secret political empire.
Let's start with the misunderstanding -- or the deliberate expansion -- of the term "dark money."
Coined in October 2010 by Bill Allison, editorial director at the Sunlight Foundation, "dark money" was meant to describe the funds spent on elections and election-related issue ads by political nonprofits that are not required to disclose the names of their donors. This money skyrocketed following the Supreme Court's 2010 Citizens United decision.
The term "dark money" does not apply, however, to every nonprofit that does not disclose its donors -- not even to every nondisclosing nonprofit with political goals, broadly speaking, on the left or the right.
"Cato [Institute], Heritage [Foundation] and Center for American Progress aren't dark money groups, and neither is the March of Dimes, which also does not disclose donors," Allison said via email. "I think of Dark Money as the money from undisclosed donors spent to influence the outcome of an election."
What kinds of nonprofits does the term cover? Mainly, "social welfare" nonprofits (organized under section 501(c)(4) of the tax code) and trade associations (organized under section 501(c)(6)), when they spend money to influence electoral outcomes. It can also cover shell corporations that spend on elections and have no other apparent purpose.
Those not included under the "dark money" moniker: public interest nonprofits (organized under section 501(c)(3)), which may be involved in shaping policy but are forbidden to engage in electoral activity, and labor unions (organized under section 501(c)(5)), which can participate in elections but must disclose their donors to the Labor Department.
The Koch brothers run most of their political empire through a network of 501(c)(4) and 501(c)(6) nonprofits, the majority of which spend money directly on elections or fund those that do.