The Quran

وَهُوَ الَّذي خَلَقَ السَّماواتِ وَالأَرضَ بِالحَقِّ ۖ وَيَومَ يَقولُ كُن فَيَكونُ ۚ قَولُهُ الحَقُّ ۚ وَلَهُ المُلكُ يَومَ يُنفَخُ فِي الصّورِ ۚ عالِمُ الغَيبِ وَالشَّهادَةِ ۚ وَهُوَ الحَكيمُ الخَبيرُ

(Saheeh International)

And it is He who created the heavens and earth in truth. And the day [i.e., whenever] He says, "Be," and it is, His word is the truth.¹ And His is the dominion [on] the Day the Horn is blown. [He is] Knower of the unseen² and the witnessed;³ and He is the Wise, the Acquainted.

When interpreted as the "Day" (of resurrection), the sentence would read: "And the Day He says, 'Be,' and it is, His word will be the truth."
That which is absent, invisible, or beyond the perception of the senses or of the mind and therefore is unknown to man, except for what Allāh chooses to reveal.
What is present, visible and known to man. The knowledge of Allāh (subḥānahu wa ta‘ālā) includes the reality of all things and all occurrences, no matter how they might appear to human beings.