An expansive network of “shadowy” dark money donors has grown to rival the influence of the conservative Koch brothers -- pumping millions into left-wing causes ranging from health care to climate change to abortion -- all while flying well under the radar of public scrutiny, according to an explosive new report obtained by Fox News. The report, by conservative watchdog Capital Research Center, describes a band of nonprofits operating under the banner of Washington-based philanthropy company Arabella Advisors. Those “pop up groups” are housed in four Arabella-controlled “sister” nonprofits, according to the report: the New Venture Fund, Sixteen Thirty Fund,...