How to reclaim unoccupied disk space with Disk Utility? I have a 2015 MacBook Pro which I tried to set up for dual-booting OS X and Linux. It resulted in some unclaimed free space on my SSD, see screenshot below.

Unfortunately, I am unable to reclaim that free space: It is grayed out and there seems to be nothing in Disk Utility that I can use to change that.
The answer is in the screenshot you posted. "To resize a partition on the selected disk, drag its resize control and click Apply." You should be able to just increase the size of the Macintosh HD partition to use the free space. I would recommend backing up first to be safe.