Making a fully solid STL without any empty gaps inside it

[miozeriux]

Hello guys, I'm new to the forum, never really had problems with 3d printing before, but now I am wondering what is the best way to make this STL truly full and solid. I attached picture of my stl, as you can see it has some weird stuff going inside it, is there any way to clean that stuff inside and make it a full solid mesh? I want to print it and if I drill a hole in it, it must be solid. Any help is much much appreciated!!