Library previews are very dark

First, the preview architecture in Develop is unique to all other modules and the most accurate preview of the data. You should always view previews there and elsewhere at 1:1 or greater and when comparing, use the same zoom for each. 
It is normal to see slight differences at 1:1 but if really excessive, the first thing to try is to disable GPU in preferences; better? If not, you might have a corrupted display profile. Recalibrate the display or whatever you usually do to create a display profile. When given the option to build this profile, pick Version 2 (not V4) and Matrix not LUT. As to viewing Develop module and Library and outside of LR, it helps to soft proof in Develop using the same profile used in Library (Adobe RGB (1998)) OR the profile selected in Export for the edited image. 

If non of that works, reset your LR preferences. https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html
Also see: https://community.adobe.com/t5/lightroom-classic-discussions/important-steps-for-manually-resetting-preferences-omitted-from-help-article/td-p/12972518

Quit Lightroom Classic.
macOS: Press and hold the Shift + Option keys.
Windows: Press and hold the Shift + Alt keys.
While holding down the keys, launch Lightroom Classic.
When asked, select "Yes".