Antonyms for the word "make lighter" would be "make heavier," "burden," "weigh down," "oppress," "complicate," or "intensify." These antonyms suggest the opposite of lightening a situation or object, instead increasing its weight or difficulty. Making something heavier can be seen as adding to the load, making it more challenging or burdensome. This concept is not only applicable to physical objects but also applies to events or feelings. For example, you can "make lighter" a tense situation by using humor, whereas "making heavier" means making the situation more stressful or serious. So, it is essential to choose your words carefully, and sometimes using the antonyms of "make lighter" may better suit the context.