My Macbook Pro is not using the entire display on my dell u3415w (black borders) ?

What are your settings in Sys Prefs > Displays?

I normally have the "best for Dell" ticked, but chose the scaled settings purposely to show the different resolution options. My machine is an MBP. Could it be that you need to adjust those settings or choose "best for your display"? And, what is the resolution on your external display compated to the ones showing in your Prefs?

Ok, I looked at the specs of both - your MBP shows this:

Your Dell is 3440x1440 (21:9) which is quite different (height vs. width). Your resolution is not mentioned in the above screenshot, so I do not know if you would ever get it to display the entire monitor.

Have you tried adjusting the actual Monitor's Aspect Ratio settings.

Looking around the monitor's manual, it appears a cause of black bars is possibly an incorrectly set Aspect Ratio.

https://downloads.dell.com/manuals/all-products/esuprt_display_projector/esuprt_ display/dell-u3415w-monitor_user%27s%20g…

You can also run the Monitor's Built in Diagnostics, and see what they say.

The resolutions mentioned for the Mac are the Maximum the Macs can support with the Specific number of Monitors mentioned. The Mac is not in any way limited to just those resolutions however.

It is able to correctly output all the other supported resolutions.

What is the native resolution of your MBP - the 2560 x 1080? I'm no expert in this, but am guessing that the Dell will only display the maximum native resolution of your MBP (despite the settings). But, as I said, I'm no expert in this. Hopefully someone else will chime in.

Are you mirroring the display? If so deselect mirroring and use extended display mode. Mirroring mirrors the resolution as well as the desktop.

I have a "best for Dell" because my external monitor is a Dell. How about you check your Dell monitor for its native resolution and see if you can pick it in the "scaled" options.