Dark Oak trees are not growing indoors in minecraft

13 blocks sometimes can even be too low for these trees (most of the time actually) but it could also be the light level.

These are spruce saplings, not dark oak saplings. Spruce needs 14 blocks above the saplings (15 including saplings) to grow; even then most growth attempts will fail; large spruce can grow even up to 30 blocks tall, so only about 1 in 30 attempts will succeed with space of 15 blocks (and none with 13).

Dark Oak would fit in the space you have prepared just fine, but dark oak saplings look like this: