Not only anaerobic, the empty spaces should be sterile!

You have two major cavities in your body: your gut area on the front and the area on your back from your butt up to your brain.

There is no circulation, everything moves by diffusion.

Your organs are lined in a membrane called "peritoneum". It sweats out a special liquid called "peritoneal fluid." You can think of it as fancy lubrication for inside human bodies.

The peritoneal liquid in the spaces between organs is really small volume, maybe 50 ml in a human. It contains neutrophils, mononuclear cells, eosinophils, macrophages, lymphocytes, desquamated mesothelial cells, and an average of 3.0 g/mL of protein. -> blah blah blah that is mostly immune system stuff and dead outer skin cells from your organs.

Electrolytes and small molecules can diffuse into/out of your blood, so composition of those is similar enough.

Overall: it's mostly blood liquid minus the blood cells, plus some immune system stuff.