The darkest igneous rock is generally considered to be gabbro.

Here's why:

* Composition: Gabbro is a mafic igneous rock, meaning it is rich in magnesium and iron. These minerals, like pyroxene and olivine, are dark in color.

* Grain size: Gabbro is coarse-grained, meaning its crystals are relatively large. This allows the dark minerals to be more visible.

While other dark igneous rocks exist, like basalt and peridotite, gabbro tends to be the darkest due to its mineral composition and grain size.