The following contains spoilers for Spider-Man: Across the Spider-Verse, now in theaters.
In Spider-Man: Across the Spider-Verse, audiences, as well as Miles Morales, learn he was never supposed to become Spider-Man. The spider that bit him is from a different universe, one that now has no Spider-Man. According to Miguel O'Hara, there is only meant to be one Spider-Man per universe, and the spider biting Miles prevented another universe from having a Spider-Man and put Miles in a position where Peter Parker had to save him, thus costing him his life. Because of all of this, Miles' Spider-Man is an anomaly.
The theory that there is supposed to only be one Spider-Person per universe undermines the notion that anyone can wear the mask, and Miles proves once again he is Spider-Man, regardless of what others say. This theory, while flawed, does at least provide an in-universe explanation for why Silk is not in Across the Spider-Verse; however, Silk could also be proof that Miguel's theory about anomalies is bogus.
Why Is Silk Not in Across the Spider-Verse?
In the comics, Silk, aka Cindy Moon, is bitten by the same radioactive spider that bit Peter Parker; however, instead of becoming a hero out of the gate, Cindy is sent into a bunker thanks to Ezekiel Sims. Several years later, Peter would release her from the bunker, and the two would have a special connection from there.
Based on the comics, Silk is proof there can be more than one Spider-Person per universe. If Across the Spider-Verse were to include her, her origin would need a serious facelift in order to keep Miguel's theory intact. However, this does not mean that Silk cannot be included later in Spider-Man: Beyond the Spider-Verse, further proving to Miguel and the Spider-Society that the anomaly theory used against Miles is incorrect.
Silk Is Proof the Anomaly Theory Is Bogus
With Gwen Stacy putting together a team at the end of Across the Spider-Verse, it's possible other Spider-Persons -- specifically those Miguel deems anomalies -- could be recruited to help Miles and his family. After all, while Miguel does not want Miles to be part of the Spider-Society, he also does not intervene with Miles' work as Spider-Man until The Spot starts to mess with the multiverse. It's possible that Miguel has left other anomalies -- like Silk -- alone unless they further interfere with the multiverse.
If Silk is out there, then she is like Miles -- a Spider-Person existing in a universe that already had a Spider-Man, thus proving Miguel incorrect. Along with that, Miguel appears to be incorrect about anomalies not being "real" Spider-Persons based on his theories about canon events. While Miguel insists Miles was never meant to be Spider-Man, he experiences a canon event -- the death of a loved one -- that every Spider-Person is "destined" to go through.
This could mean more than one Spider-Person can exist per universe as they still experience these defining events, or this means canon events are more flexible than anticipated as Miles' universe has survived two Spider-Men following this predestined path. Either way, it is further proof that Miguel's theories about anomalies and canon events are not as sound as he wants them to be, and Gwen defies his theories even more.
By the end of Across the Spider-Verse, Gwen realizes there are ways to work around and even prevent canon events, such as her father resigning from being police captain. With this in mind, Gwen knows they can keep a universe in one piece without sacrificing the lives of others. Between this revelation, Miles consistently proving he is Spider-Man, and the possibility of other Spider-Anomalies -- like Silk -- existing, Miguel's theories about anomalies and canon events are not as solid as the Spider-Society once believed.
To learn more about Miguel's multiverse theories, Spider-Man: Across the Spider-Verse is now playing in theaters.