10 Things That Make No Sense In Doctor Who
3. The Flux Is Stopped By Matter, Except When It Isn't (The Vanquishers)
Series 13 of Doctor Who revolved around the Flux, an enormous hungry cloud that devoured anything and everything in its path.
As the series progresses, we learn that the Flux is composed of anti-matter, and that it was created by Tecteun as part of a moustache-twirling evil scheme. The Thirteenth Doctor soon determines that the only way to stop the Flux is to, essentially, "feed" it, putting enough matter in its way to combat the anti-matter.
It's a sound theory, and even better, finale episode The Vanquishers shows us that it works, with the Doctor using the endless space inside a Passenger to absorb every last drop of the Flux.
The only problem here is that, if matter is what halts the Flux, then why wasn't it slowed down (or stopped) earlier in the series? In the very first episode, we see it eat countless planets, and billions of life forms. It also rips through Vinder's space station, and later on, it even has a munch on some Dalek and Cyberman fleets.
The Sontarans seem to think that those fleets will stop the Flux all by themselves, so it doesn't make sense that a bunch of planets wouldn't also get the job done.