The fluid inside chloroplasts that surrounds the thylakoids is called the stroma.
The stroma is a protein-rich, alkaline, aqueous compartment enclosed by the chloroplast’s inner membrane. Thylakoids—flattened membrane sacs that often form stacks called grana—are suspended within it. The light-dependent reactions occur in the thylakoid membranes, while many carbon-fixation reactions occur in the surrounding stroma.
The Calvin cycle takes place in the stroma. It uses ATP and NADPH made by the light-dependent reactions to incorporate carbon dioxide into organic molecules and eventually produce G3P, a three-carbon sugar phosphate. The stroma also contains chloroplast DNA, chloroplast ribosomes, starch granules, and enzymes involved in chloroplast metabolism.
The main confusion is between stroma and lumen. The stroma is outside the thylakoids, whereas the thylakoid lumen is the aqueous space enclosed inside each thylakoid. Cytosol is the fluid outside the entire cell’s organelles, and matrix is the corresponding internal fluid of mitochondria, not chloroplasts.