It's sometimes called "the most famous chord in rock n' roll." I have wondered about this question for thirty-four years (all this time I'd been thinking it is an odd hybrid G7/9/13). Here is a history of thought on the controversy, including a list of nominated chords. It now turns out there is an answer. A mathematician applied Fourier transforms to break the sound into its constituent parts. Here's the bottom line:
The Beatles producer [George Martin] added a piano chord that included an F note,
impossible to play with the other notes on the guitar. The resulting
chord was completely different than anything found in songbooks and
scores for the song, which is one reason why Dr. Brown’s findings
garnered international attention. He laughs that he may be the only
mathematician ever to be published in Guitar Player magazine.
Here is a pdf of the researcher's findings. I thank Eric H. for the pointer.