If you're referring to white hard tissue found mainly near the stem end it's called the tomato core and is found in some OP vareities but not others.

Which of the varieties that you're growing are showing it?

Carolyn, assuming you're talking about in the flesh of the tomato, not the white hard tissue that can be found under stinkbug bites just undernath where stinkbugs have bitten through the tomato skin.

Could be sun scald.

If it's not the normal core or the stinkbug damage that Carolyn mentioned, it could also be a potassium deficiency. If it's a potassium problem, there should be other symptoms, like leaf problems. Here are some pictures.

Here is a link that might be useful: Potassium deficiency