Prediction for data the model haven’t seen?

Hey, I have dataset of brain tumors mri images with three type of tumors, when i trained a u-net model the results came out (79%) dicecoef.the problem i am facing is if i added healthy brain images to the database the results came almost smiller 79%, but when i plot the results for healthy images, the result should be blank dark but they show like this https://imgur.com/a/jXbLUMt .

how can i fix this problem ?

and in general question how to train a model to handel images not trained on,like if a model trained on classification of images of cats and dogs and presented an image of tree shouldn’t the model give results of unkonw or something familier?