Where is the space ? I have no clue without more info

What is their depth? How many are there?

To clarify, their depth is 24inches and height is 44in and width is 24. There are two. The space is in my bedroom.

I would have someone install shelves, probably adjustable ones, in the spaces. Figure out what you might want to put there--boxes, baskets, books, antique quilts...then plan the shelving to fit. You could even put in drawers for your underwear and socks.

what are we looking at? looks like a closet. show us some context

Another question might be how high up they are? do they start near the floor or is the top over your head?

if they are not too high, I like AnnKh‘s drawer ideas. You could hire a local carpenter to build drawers inside them (metal full extension glides)

if they are higher, you could block off the back 6-9 inches. Reduce the depth to ~18” and install 12-16” deep shelves. you could space the shelves So that IKEA cube boxes could fit.

what is in the rest of the wall? do you have a false wall or something? Or are these cabinets that protrude? Maybe an option would be to remove the false wall and make the room bigger (unlikely but you have to wonder what is in the rest of that space) Or just removing them if they are hanging cabinets.

If you just want to use these for behind closet door storage and are not trying to make them appear as a finished feature:

For supporting each shelf you could use three 1"x4" boards -- each side front to back and another the full width of the back between the boards on the side. Keep the tops of all three boards level, nailing them to the wall at the location of studs.

For each shelf, you could use two ten inch boards. If, however, you're adding one rod and one shelf above it, use only half that depth shelf against the back wall.

Space the shelves as far apart as you will need to make room for whatever height items you want to store.

Keeping closet doors over them means less dusting.

For a finished visible unit for displaying things, use finishing grade lumber and build the shelving unit the width of the doorway and the depth of the space, slide it into the niche and then add trim boards all around covering the space between opening and shelving unit.

A 24" depth may not be deep enough for adult clothes but if you want clothes rods in any closet, add the three boards as if for a shelf and add hardware for holding the rods.

https://www.harneyhardware.com/products/shower-rod-mounting-brackets-stainless-steel-pair-packed-satin-stainless-steel-53365?gclid=EAIaIQobChMI_aqkxNP46QIVAYiGCh3H_g2rEAQYAiABEgKel_D_BwE

If you are not DIY inclined and just want shelving within the closet:

https://www.acehardware.com/departments/storage-and-organization/shelving-and-shelving-units/shelving-units/5939038?x429=true&gclid=EAIaIQobChMIx8rTyNj46QIVI4VaBR3FoQL3EAQYAiABEgK9X_D_BwE&gclsrc=aw.ds