Darkness

Darkness is a visually-impairing obstacle recurring throughout the Super Mario franchise. Darkness is a large black mass that covers most of the levels that it is present in. However, entities such as enemies, players, items, and coins remain somewhat visible, surrounded by faint light.

In most cases, after touching a Super Star, darkness disappears until the end of the player(s) invincibility duration.

New Super Mario Bros. Wii
Darkness appears in multiple levels of New Super Mario Bros. Wii. It is first present in World 2-3, where it is warded off by Fire Bros, Fire Piranha Plants, and Fire Snakes.

New Super Mario Bros. U / New Super Luigi U / New Super Mario Bros. U Deluxe
Darkness returns in New Super Mario Bros. U, New Super Luigi U, and New Super Mario Bros. U Deluxe, serving as the same visual hazard in all three games. The player(s) are able to temporarily push back the darkness with Glowing Baby Yoshis.