Darkened Photos in Develop Module

What you are seeing is probably simply the actual camera raw rendering being applied. In library you at first see a jpeg preview generated by the camera. Then in develop, Lightroom shows its own rendering of the raw data using its own engine. For many cameras you can select a camera matching profile (typically called camera default) and get similar rendering. That won't work if you use any of the HDR modes in camera. Nikon calls this dynamic lighting and canon has several names including highlight priority. What those modes do is underexpose your image but the jpeg engine in camera compensates for it and uses a HDR technique to bring back the image. Lightroom doesn't know about this and will render too dark in that case. There is actually now a new way to work with this. You can also use the tricks in this article: https://community.adobe.com/t5/lightroom-classic/lightroom-classic-set-defaults-for-raw-file-import/td-p/10925693?page=1 if you set the default for your camera to "camera settings" and the feature is available for your camera, Lightroom Classic will try to compensate for these settings and get you a rendring not too far from the in-camera jpeg even if you use the dynamic modes. Not perfect but it works.

If this isn't what is happening, the question becomes: "How are you calibrating your display?" This is often an issue with the monitor profile so calibrating using good calibration hardware would help.