I just found out that there is an extracellular matrix. I had imagined that the tissues, organs, and the whole body was a bone skeleton with cells attached to the bones and to each other. I imagined the cells to be joined directly to each other like atoms in a crystal.
In fact, most of the cells are not adjacent to each other. Rather the cells (except blood cells) sit in a solid matrix that is a mixture of proteins, especially collagen, which is like a plastic or hardened glue or rubber. The matrix is secreted by the cells, and holds them in place and connects them to each other, and separates them from each other. In bone, the extracellular matrix is strong, rigid and hard, while in cartilage it is strong but flexible like some types of rubber or plastic. In the brain it is very soft. The differences in rigidity, strength and hardness come from different mixes and densities of proteins, except in the case of bone which has a lot of calcium compounds (minerals) in the extracellular matrix (as an aside, for the sake of completeness, I'll just mention that some say the blood is a connective tissue, and that blood plasma is the extracellular matrix of the blood).
Finding out (so recently) about the existence of the extracellular matrix, for me has been absolutely amazing, and it has changed how I look at my own body.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2995612/ (which has a lot of information about the extracellular matrix including a colorful and impressively detailed giant poster that you can magnify repeatedly) says, "Collagen is the most abundant fibrous protein within the interstitial ECM and constitutes up to 30% of the total protein mass of a multicellular animal. Collagens, which constitute the main structural element of the ECM, provide tensile strength, regulate cell adhesion, support chemotaxis and migration, and direct tissue development (Rozario and DeSimone, 2010)." but doesn't say how humans compare to the multicellular animal with 30% of the total protein mass being collagen, nor what the total mass of that collagen would be.
https://biology-pages.info/E/ECM.html contains the closest to an answer that I've found, though it speaks of 'bulk' and not mass and some multicellular organisms and not humans :"While it is true that all living things are made of cells, that is only part of the story. Most of the cells in multicellular organisms are surrounded by a complex mixture of nonliving material that makes up the extracellular matrix (ECM). In some cases, the ECM accounts for more of the organism's bulk than its cells."
So my question is: How much extracellular matrix is there in the human body? I leave it to you to decide whether blood plasma is extracellular matrix and similar issues. Also it's up to you how you want to quantify the amount, whether by mass (my preference), volume, or some other measure. Also, it's up to you how precise and quantitative (my preference) or approximate or qualitative (that could also be good) your answer is. My aim is to see, in my mind's eye, the human body as it really is.