What does evil weigh?
The question in the title may seem nonsensical, like what is the melting point of the number three? How can physical properties be applied to abstract concepts?
Therefore, the only reasonable response, you say, is “nothing!”
And you would be correct, but not for the reasons you think.
Saint Augustine concluded, after examining the nature of evil, in his famous Confessions:
For you evil does not exist at all, and not only for you but for your created universe, because there is nothing outside it which could break in and destroy the order which you have imposed upon it" (Augustine, Confessions, 7.13)
Thus, while we can make the argument that the number three exists, evil does not exist at all.
Now, Augustine is not arguing as a Buddhist might, that there is no good or evil, far from it. Rather, he argues that since God created all things that exist, even the number 3, and God only creates that which is good, evil cannot exist.
When we see acts around us and label them as evil, what we are pointing to is the absence of good. Thus, evil is like darkness. Darkness is the absence of light, not a thing in itself. While light is very much real, darkness is not something we can measure except by noting that we have not measured light.
If you want to ask how bright darkness is or how much energy darkness has, the answer is nothing, not because darkness is some immaterial thing but rather because it is not a thing at all.
Such questions can only be meaningful when applied to the light.
Like light, good, on the other hand, is a real thing. If all creation is good, in fact, then the question “what does good weigh?” has an answer: the weight of everything.
The observable universe is estimated to have about 10 to the 53rd power kilograms of ordinary (non-dark) matter. Since this does not include any matter that is beyond the observable horizon, it is impossible to know how much good weighs since it includes everything beyond as well. It could well be infinite.
Even the matter comprising an evil person, a terrible dictator, is in itself good.
Indeed, according to Augustine's theology, even the immaterial being of the Devil himself is good, since he too is God’s creation.
This is why studying the physical sciences, understanding how things that exist work, is itself a good because it aims to understand what is good. To understand the universe is to pursue truth, which is always good.
On the other hand, to impede understanding or replace it with lies to satisfy greed, ideology, or one’s ego is the opposite. Anything that promotes a lie is inherently evil.
Just as light and good are real, while their supposed opposites, darkness and evil, are not, truth is real, but lies are not. For a lie is merely the absence of truth. It is not a thing in itself.
That is why we “find truth” but we “invent lies”.