Ethan Kleinberg explains il y a, from Generation Existential.
For Levinas, the structure of Heidegger’s being-towards-death misses the essential point of the il y a because Heidegger’s structure places the emphasis on finitude. What is horrifying about the il y a is not anything like finitude but the fact that it never ends and goes on without us. The il y a is precisely that which deprives us of our ownmost possibility as manifested in being-towards-death. In the il y a: "It is subjectivity, the power of private existence, that is stripped from the subject in the horror. The subject is de-personalized." What is horrifying is not the possibility of death as finitude but the impossibility of death in infinity. If being-towards-death is the possibility of no longer being possible, the il y a is the negation, the impossibility, of that possibility. "It is, if we can say this, the impossibility of death, the universality of existence even in its own annihilation”. The horror of death does not lie in the finitude but in the realization that anonymous being goes on infinitely. In this sense, the il y a is the negation of negation. Even after death it is. The il y a is pure being. It is without world and without time. It is the realization that even where there is nothing, there is.
The il y a is the recognition of being in all its strangeness and alterity, as that which is beyond representation or localization and thus is completely beyond our control. We do not observe the il y a, but rather it observes us. This is why "our relation with the il y a is horror. We have already noted its insinuation in the night like an indeterminate menace that comes from a space that is disengaged from space’s function as the receptacle of objects and the access of beings". The il y a is not representable because it does not have the properties of an object it is outside time because it is infinite. It is that which is completely beyond us and yet it is always there in front of us.