Dark is not necessarily graphic or grotesque.
Gamers need to stop conflating these feels.
Dark can also be... existential. Being on an empty planet with nothing but the mako and your two companions while the stars and planets loom ahead and the sound of wind howling - air consisting of foreign compositions. A hostile nature you can't possibly find on your home world.
This is dark. No, there aren't any beheadings or brood mothers, but it feels... philosophically disquieting to my soul all the same. I like that.
But it's not. Because you're not "actually" on an empty planet with nothing but the Mako and your two companions and the sound of the wind. You're part of an intergalactic crew you're actively in radio contact, making the equivalent of a trip to the country-side. This is a purely personal experience on your part, but I absolutely contest that there's anything existential about it.
From the point of view of the characters, this is an incredibly mundane phenomena in a world absolutely rife and teeming with non-human life. From our player perspective, again, same thing.
I don't disagree that dread can be existential. I just disagree that ME1 comes within a hundred thousand feet of an existential theme. The closest we see is the absolutely brilliant "I remember me". Now that is a dark scene, but it's a rare moment, and it's dark precisely for the reasons I point out: it's all about disempowering the protagonist, and everything about that scene is about powerlessness and abuse. Shepard's lack of power is stressed - careful approach, repeated all done by dialogue, constant stories of abuse and a lack of power, etc.