The light-independent reactions of photosynthesis occur in the chloroplast stroma.
The stroma is the aqueous, enzyme-rich fluid surrounding the thylakoid membranes and grana. It contains the enzymes that carry out the Calvin cycle, including the reactions that fix carbon dioxide and convert it into carbohydrate precursors. The light-dependent reactions, by contrast, occur in the thylakoid membrane.
Although these reactions are often called “dark reactions,” that name can mislead. They do not require photons directly, but they depend on ATP and NADPH made by the light-dependent reactions. In living plants, the Calvin cycle is commonly active when light is available because those energy-rich molecules are continually replenished.
The stroma is not an empty space. It also contains chloroplast DNA, ribosomes, and machinery for making some chloroplast proteins. A useful distinction is that the thylakoid lumen is inside the membrane sacs, while the stroma is the surrounding fluid outside them.