That Which Is, also called Creation, the Heavens, the Universe, Meleth, and Meldad is all of existence: all forms of matter and energy, from motes of dust to stars and galaxies.
Here is the beginning of the universe as described in the Rogondēndāl:
“But then from out of the Void, a new thing was birthed: That Which Is; and it was named Creation, the first to have ever been. Its offspring were the dust and the stars, and they poured out into the Void, and it was made not void. Light shone from the stars, which were as lamps in the darkness, and from the dust came the matters to build the lands and fields of Creation’s kingdom. Thusly was Meleth, the realm of all existence, wrought.
The Kembar were birthed far away amidst the stars of Creation. For eons they explored it, until they decided to create and settle a world of their own: Nīmlad. The shape, duration, extent, and contents of That Which Is are not known to the Children, whose lore of it contains only that which the Kembar have told them. Magic is essentially the ability to command and change the physical properties of That Which Is to a very limited degree.
Outside the bounds of Creation lies its opposite: That Which Is Not, more commonly referred to as the Void.
Etymology
The Paternic term Meleth technically refers only to outer space, but speakers use it as a metonym for the whole universe extremely frequently.
The proper term for the universe as a whole is Meldad, an elliptical noun that means “Heaven and Earth”.