REUTERS/Rick Wilking
But his presence was certainly felt.
He was, quite literally, right down the road, hosting his own event. He dominated the social-media conversation. And from the minute the debate started, Trump's rivals were forced to confront Trump's absence amid a high-profile boycott.
"For one night, he beat Fox News," wrote NBC senior political editor Mark Murray on Twitter.
The debate began with Fox News host Megyn Kelly questioning Sen. Ted Cruz (R-Texas), whom she asked how Trump's decision to boycott the debate reflected on his view of Iowa voters.
Cruz immediately mocked Trump's aggressive, below-the-belt rhetorical punches.
"Let me say that I'm a maniac and everyone on this stage is stupid fat and ugly, and Ben, you're a terrible surgeon," Cruz said, gesturing at retired neurosurgeon Ben Carson.
"Now that we've got the Donald Trump portion out of the way, I want to thank everyone here for showing the men and women of Iowa the respect to show up and make the case to the people of this state why each of us would make the best commander in chief," he continued. "There's a difference between personal insults and attacks - between going into the mud with ad-hominems and focusing on issues and substance."
Later in the debate, Cruz managed to sneak in a jab at Trump while complaining about what Cruz asserted were unfairly pointed questions.
"Gosh, if you guys ask me one more mean question, I may have to leave the stage," Cruz quipped.
Responding to the first question directed at him, meanwhile, former Florida Gov. Jeb Bush (R) joked that he "missed" Trump.
"I kind of miss Donald Trump. He was a little teddy bear to me," said Bush, who has famously feuded with Trump on the campaign trail. "We always had a loving relationship during these debates and in between, in the tweets. I kind of miss him. I wish he was here. Everyone else was in the witness-protection program when I went after him on behalf of what the Republican cause should be: conservative principles, limited government."
Though Trump was mentioned and referred to occasionally throughout the debate, he dominated the online conversation surrounding it.
Halfway through the debate, Twitter said 37% of the conversation about candidates was dedicated to Trump. Cruz placed second with 18%.
Trump also garnered the most searches in Google.
And Trump's alternative event, his fundraiser for veterans held at nearby Drake University in Des Moines, also drew plenty of attention.
Former Arkansas Gov. Mike Huckabee (R) and Pennsylvania Sen. Rick Santorum (R) - the respective 2008 and 2012 winners of the Iowa GOP caucus - both showed up after participating in the lower-tier, "undercard" debate. They spoke in support of the fundraiser (but not of Trump's candidacy) from behind a podium with a large "TRUMP" placard.
Trump had veteran John Wayne Walding - who lost a leg in battle and proceeded to reattach his leg and return fire for four hours during a mission in Iraq - give a lengthy speech about the problems veterans face upon returning home.
This vet, John Wayne Walding, is currently speaking at the Trump rally https://t.co/lXYiPkcixU pic.twitter.com/t3FY2pL3Jc
- Betsy Woodruff (@woodruffbets) January 29, 2016
An unbelievable night in Iowa with our great Veterans! We raised $6,000,000.00 while the politicians talked! #GOPDebate
- Donald J. Trump (@realDonaldTrump) January 29, 2016
For his part, Trump mostly had positive things to say about Fox. But he did throw in a quick jab at the debate, which he boycotted after complaining about supposed unfair treatment by the network and one of its moderators, Megyn Kelly.
"Isn't this better than the debate that's going on?" he said. "Right? They're all sleeping. They're all sleeping. Everybody."