Allah says in the Qur’an:
مَالِكِ يَوْمِ الدِّينِ
You might ask, why do rabb, Ar-Rahmaan, Ar-Raheem, and Maalik all have kasra on them? Why not dumma (the default), or fatha?
The answer is that, wallahu ‘alim, they are na’at (adjactives) for Allah–words that describe His greatness.
For example, Ar-Rahmaan means “the source of mercy” (that’s why, scholars say, you cannot name your son Rahman). Ar-Raheem is the one who shows mercy to the creation.
And rabbil ‘alamiyn means “Lord of the Worlds” (yes, worlds, not world). And Maaliki Yawmid-Deen means king or soverign of the Day of Recompense.
Inshallah we’ll post more details (tafseer) on each of these aspects in the future.