From a Sufi Perspective
We call God “Allah” because there is no gender implied in this Arabic appellation. Allah can have no gender because that would be a limiting factor, and Allah can have no limits. We believe that God is omniscient, omnipotent, and is the uncaused Cause of all creation. Allah is without limit in every dimension, infinitely and eternally, the formless Creator of all forms. Allah is neither created nor can He be destroyed.
The one thing in the universe we know of that cannot be created or destroyed is energy. Energy can be understood as the capacity to do work, as the force that is behind all action, and as action itself. We know energy as a kind of light/force, as that which radiates and that which is radiated. It is both substance and movement. Sufis believe that Allah is the fundamental energy which has existed before all time, because Allah is both uncreated and the Creator. We understand Allah as the conscious energy of the universe, the One who contains infinite knowledge and power to act in whatever way He wills.
Allah defines Himself in the Holy Quran as “…the Light of the heavens and the earth” (24:35). Just as we understand that light is energy, we conceive of Allah as energy. Allah as pure energy is formless, but contains the potential to create all forms. Allah’s power is at once the energy used to create all forms, and the energy/substance of forms. For Sufis, Allah’s inherent quality of awareness, His comprehensive and eternal consciousness, is another aspect of the “Light” by which He describes Himself. There can be no god but Allah, because the existence of any other power would limit God, Who is without any limit whatsoever.
Allah is the Light of the heavens and the earth. The analogy of His light is as a niche, and within it, a lamp. The lamp is enclosed in a glass. The glass is like a shining star. Lit from a blessed tree, an olive neither of the East nor of the West, whose oil is almost luminous, though no fire touched it. Light upon light. Allah guides unto His light whom He will. And Allah speaks to mankind in allegories, and Allah is Knower of all things. (Qur'an 24:35)
With further investigation into the concept of Allah as energy, another notion emerges. If Allah is both the substance and the Creator of creation, there can be nothing else but Allah. There can be no place where Allah is not because that would imply that something outside of Allah exists. Thus, from the Sufi perpective, nothing exists but Allah.