How sad. People concerned about interference in elections aren't people who lost - it's not a partisan issue whether Russian intelligence or the FBI is interfering in US elections.
> If they messed with enough voting machines to make a difference, NSA etc would know about it
That's completely theoretical and U.S. intelligence's real track record isn't nearly that good. There was Pearl Harbor, the Bay of Pigs, the collapse of the Soviet Union, 9/11, being unaware of Saddam Hussein's WMD programs before the first Gulf War, a false positive on Hussein's WMD programs for the second one, stolen nuclear warhead and stealth fighter plans (among many others); major hacks of many major government facilities including Dept of State, OPM (which include files on undercover spies), and many more; and finally, during the election, the successful attack on the DNC. - Those are just the highlights. I don't feel as confident as you do.
Finally, sometimes government doesn't tell the public everything. President Johnson knew that Nixon was privately negotiating with the North Vietnamese, undermining U.S. foreign policy, but chose to say nothing because (AFIAK) he thought it would be too disruptive.
None of that is evidence that something happened, of course; only that we can't rely on the NSA to detect everything and notify the public about it.