Space between first staff is horrendously large

Press CTRL+A, and then CTRL+R, and the space is gone ...

I guess Strg is the label on some keyboards, but for most its Ctrl. Ctrl+A selects all, Ctrl+R resets. Or on Mac keyboards, use Cmd.

As for how it happened, it seems you have an empty dynamic attached to measure 7, and that empty dynamic has been dragged way out of position, and MuseScore is dutifully making room for it. On the assumption you don't want that empty dynamic there at all, better to simply delete it. To find it, click the first note (Eb) of the measure, then press Alt+Right to move to the next element, and you'll see the status bar reports it's a dynamic, and the Inspector shows it has a large offset applied. Just press Delete and it's gone, and with it, the extra space.