I'm new to Inkscape, and in all of my works the layers are see-through despite having 100% opacity. I've already tried looking online for solutions, but nothing has fixed it. Can someone help?

One likely solution is that the display mode isn't set to normal.

Else, remember that opacity can be set for layers, groups, individual paths or objects, and colors. Check at each level.

You can use object panel to help you : the semi transparent objects have a small striped square icon.

On the screenshot below, you can see where semitransparency occurs : I duplicated group g1 and modified opacity (50% each time) at different levels. I intentionnaly placed each group beside its description in object panel for ease.

• In layer 1, group g1, there is no transparency : it's the original group
• In layer 1, group g2, half opacity concerns rect6 and circle6
• In layer 1, group g3 has half opacity : everything inside this group looks half transparent
• In layer 2, the transparency concerns the layer then everything inside this layer looks half transparent
• Finally, in layer 3, I tweaked fill and stroke colors to set them a 50% alpha : there is no striped rectangle icon at any level (object, group, layer) : you can conclude that opacity concerns colors. See also that when alpha isn't set to 100 for stroke colors, fill is visible behind the stroke (if you did'nt alter fill , stroke and markers order in fill and stroke panel).