Can you provide a photo further back that shows the entire tree, including the location the branch broke off from? This looks like heart rot, which would indicate that there had been a wound of some sort that allowed fungi into the inner part of the tree. There is no way to stop the decay from slowly spreading through the tree, and there is no treatment that we recommend, other than cleaning up any jagged edges, loose bark, etc. What I can't tell from the photos is whether the decay started from a wound in the large branch, or from another wound in the trunk. That makes it difficult to assess what shape your tree is in.

If you would like to have a certified arborist look at your tree: https://www.treesaregood.org/findanarborist