The evidence was set out in a briefing by the CIA.  U.S. Senator and former Republican presidential candidate John McCain was briefed and this is his view:

"The facts are there," he said during an interview on Face the Nation Sunday. "Whether [the Russian government] intended to interfere to the degree that they were trying to elect a certain candidate, I think that's a subject of investigation. But facts are stubborn things. They did hack into this campaign."

Four high-profile U.S. Senators from both political parties issued a joint statement on the recent intelligence findings. Democrats Chuck Schumer and Jack Reed, along with Republicans John McCain and Lindsey Graham  called for an investigation into American intelligence agencies’ conclusion that Russian hacking was intended to help Trump defeat Hillary Clinton.

Simply because Donald Trump chooses to ignore the facts does not mean they do not exist.