One option is to install wainscoting on the backs of the cabinetry to give a more finished look.

Adding picture frame molding and stain it to match that great wood color. This will add an architectural element and still not add visual weight.

I would cover it with drywall and paint it to match the den walls.

I agree with painting it to match the den..the flow would be better

More options are , tile it with a thin light weight mosaic glass, or wood veneer sheets. There's also beautiful textured copper sheets, decorative textured glass, even exotic basket weave tiles in wood finishes. The choices are limitless, email me of you want information on where to look at these items .
RepurposefulThings@yahoo.com

thank you all so much. More ideas than I have thought of.

I agree with groveraxle, make it go away. You don't want them to be a focal point.

we wanted to rip it out and open the kitchen to the den but due to extensive floor damage ( brown fungus that ate the subfloor in the den) we are out of funds for a kitchen redo). i am like a lot of these ideas...as its a huge wood elephant in the room when in the den.