Git reads a global excludes file in addition to each repository's .gitignore. By default it is ~/.config/git/ignore, read automatically if it exists, so editor and OS clutter never needs to go in a project's .gitignore:

mkdir -p ~/.config/git
echo '.DS_Store' >> ~/.config/git/ignore
echo '.idea/' >> ~/.config/git/ignore

The longer story is in Two git ignore files nobody told me about.