file-match-ignore.md

If your projects directory tree looks like this:

projectName
│
└─── node_modules
│   │
│   └─── ...
│   
└─── src
│   │
│   └─── node_modules
│   │   │
│   │   └─── @subproject1
│   │   │   │
│   │   │   └─── ...
│   │   │
│   │   └─── @subproject2
│   │   │   │
│   │   │   └─── ...
...

And you need react-cosmos to track files under your own projectName/src/node_modules directory but keep ignore projectName/node_modules, you may benefit from redefining fileMatchIgnore:

'**/projectName/node_modules/**'

That way react-cosmos will ignore only projectName/node_modules.