The doctrine of creation ex nihilo is one of those ideas that is generally accepted in Western churches but is not considered essential to salvation. Among the Eastern churches, alternative views are more prevalent.
A widely accepted Orthodox idea is that of St. Gregory Palamas which that there is such a thing as "uncreated light," a real existence that can be perceived through certain mystical practices. This led to controversy because it implies that this light was certainly not created ex nihilo; indeed it was not created at all, yet it really exists in the world. While Western theology generally does not accept this idea, it has not been officially condemned as heretical.
More recently, the Russian theologian Sergei_Bulgakov wrote that the idea that God is "all in all" extends to what western thinkers call "nothing." In other words, "nothing" cannot "exist" apart from God. Accordingly, Bulgakov was willing to say such things as:
Divine being is limitless. 'Nothing' is by no means like an ocean that
flows around this being. God created the world out of Himself.
Another Russian Orthodox theologian Nicholas Berdyaev, spoke of "uncreated freedom," as an aspect of God that is essential to human nature. This too exists as a reality in our world, but was not created ex nihilo.
The above notions conform to the view that the universe or certain aspects of it were not created "ex nihilo." Orthodox thinking on this issue is diverse. For our purposes it will suffice to say that the traditional western idea of creation ex nihilo is not universally accepted per se in Greek and Russian Orthodox traditions.
Also, the LDS Church (Mormons) basically rejects the doctrine of creation ex nihilo. It is a large denomination, though not necessarily considered mainstream.
Another non-mainstream tradition, the Unification Church, holds that God created the universe out of Universal Prime Energy, an aspect of God that is the "origin of all energies and forces that allow created beings to exist."
Finally, see Theologies of Creation by Thomas Jay Oord and Mary-Jane Rubenstein for various alternatives creation ex nihilo, based both on ancient and contemporary approaches.