R. Aryeh Kaplan, Immortality, Resurrection and the Age of the Universe: A Kabbalistic View, p. 11:
It would appear, then, that the seven days of creation described in the Torah actually occurred in thought rather than deed. Of course, God's thoughts are not the same as ours, and it is possible to say that creation in thought actually refers to the creation of the spiritual counterparts of the physical world. This approach is found in a number of Hasidic sources.(See also here)
Thus, it may be that the seven days of creation took place over 15 billion years ago, before the Big Bang. This represented the creation of the spiritual infrastructure of the universe, which the Talmud refers to as "creation in thought." The universe then developed according to God's plan, guided by the spiritual infrastructure He had created. Finally, less than six thousand years ago, God created Adam as the first of a new type of being. Although human beings may have existed before Adam, he was the first to acquire a special spiritual sensitivity and be able to commune with God.
 See No'am Elimelekh, Hayyei Sarah.