Mac storage full but nothing on it? Where the space actually went

My Mac says it's full, but I have nothing on it

The short answer

The space is real. It's in places Finder never shows you: hidden Library folders, iPhone and iPad backups, attachments from Messages and Mail, files apps left behind after you deleted them, and the Trash. None of it is things you saved on purpose, which is why the Mac feels full of nothing.

The usual hiding places

System Data. Caches, browser data, chat files, developer folders. Often 50–150 GB. What's inside →
iPhone and iPad backups. A single backup can be 40 GB, and old ones are never removed. They live in ~/Library/Application Support/MobileSync/Backup.
Messages attachments. Every photo and video anyone ever sent you, kept on the Mac.
Leftovers of deleted apps. Dragging an app to the Trash leaves its data behind. Docker leaves 16 GB.
The Trash and Recently Deleted in Photos. Nothing there is freed until it's emptied.

Find it without any app

Open System Settings → General → Storage. Click the ⓘ next to each category to see its biggest items.
Open Finder → Go → Home, then press ⇧⌘. to show hidden folders.
This command lists the biggest folders in your home folder and changes nothing:
```
du -sh ~/* ~/.[!.]* 2>/dev/null | sort -h | tail -15
```