Thanks to the Citizens United Supreme Court decision, groups may spend an unlimited amount on political campaigns. And those groups don't have to reveal where they get the money they spend. That money spent without revealing the donors is called "dark money". Between January of 2010 and December of 2016, about $800 million of dar money was spent on political campaigns. And three quarters of that money ($600 million) has been spent by only 15 groups. Those 15 groups are listed above. Note that most of the money spent is by right-wing groups (who spent the money supporting Republicans).
This information is contained in a new report from Issue One called Dark Money Illuminated.