This is book 8 of the "Young Wizards" series.

This note contains spoilers for the book.

Huh, I was sure I hadn't read this. I'm well into it, and haven't remembered anything. Scary. Not, I don't think, the book's fault.

Dark matter turns out to be evidence of a plot, possibly not even by the Lone Power, to ruin everything by changing the expansion rate of the universe.

One of the symptoms is that the older wizards are losing their powers. If things go too far, everybody will lose their powers, and nobody will even remember wizardry exists. And everything will go completely to hell.

But they've got Nita and Kit and the visiting wizards (from Holiday) and Ronan (from Abroad, including his avatar of the Defender, and carrying the spear) off (folliwing Kit's dog Punch) to find the weapon they need, and meanwhile Dairine is back to the Motherworld (where she created the race of computer wizards in High Wizardry).