Is there negative marking in the dMAT exam? Should I guess if I don't know an answer?

No, no credible source confirms any negative marking on the dMAT. The official instruction for every subtest is explicit: you should guess if you do not know the answer.

Why Guessing Always Wins

A blank response earns nothing, while even a narrowed-down guess has a real chance of scoring. Unanswered questions count the same as wrong ones, so leaving an item blank is always the worse choice compared to an educated guess.

Build the Habit in Practice

Because the maths of it never changes, there is no situation where a blank beats a guess. Treat every item as worth an attempt, even a rushed one in the final seconds of a subtest.

My Advice

Make a hard rule for yourself during mocks and on test day: never leave an item blank, no matter how unsure you are. Building this habit during practice means you will not hesitate and lose time deliberating on test day itself.