What is this black bar in my dock below application icons?

I know small black dots indicate an application is open. But there is a bigger black bar that's underneath 2 icons that are not open. It doesn't matter which app I put above it. It's always in the same place. And it only shows up when the dock is on the bottom (screenshot attached).

Yes, I've changed the wallpaper background.

Yes, I've restarted the computer.

Yes, I've re-launched finder.

Yes, I know this isn't a big deal.

OS: 10.10.5 Yosemite

^Black bar

^No black bar