Vyakta or the manifest is that which is perceivable by the sense organs and mind or at least conceivable by the intellect. The universe or cosmos, i.e. the world including all physical matter and creatures with their various faculties, is vyakta (manifest). On the other hand, avyakta (unnmanifest) is the unperceivable cause of this manifest world and the creatures in it. The absolute unmanifest (param avyakta) is inconceivable even by intellect.
The Bhagavad Gita states – On the advent of day, i.e. creation (Srishti), all vyakta (s) (manifests) evolve from the avyakta, and on the arrival of night, i.e. total annihilation (pralaya), all of them are dissolved back into that very avyakta (Avyaktadvyaktayah sarva prabhavantya haragame, Ratryagama praliyante tatraivavyakta sanjnake – Bhagavad Gita VIII – 18). What is this avyakta? One may call it the creator God who creates and annihilates the world again and again. Theist philosophies like Nyaya and Vaisheshika would accept this view with a slight modification – that God does not create the manifest world from within Himself, but He does so by organizing the eternal atoms into various forms.
But Samkhya philosophy recognizes prakriti (primordial nature) as the unmanifest cause of the manifest creation and calls it avyakta. When the equilibrium of the three basic qualities (triguna) in the avyaktaprakriti is disturbed due to the proximity of the purusha (pure conscious being), reinforcement of the three gunas begins, causing the first manifestation of the prakriti as mahat (at the cosmic level) or buddhi (intellect – at an individual level); then successively the grosser manifestations take place as a result of which a vyakta (manifest) world comes into existence. In the reverse process, all merge into their respective causes, ultimately everything getting merged into avyaktaprakriti.
However, be it srashta Ishvara (creator God) or Prakriti, the absolute unmanifest (param avyaktam) is still more absolute unmanifest (param avyaktam) is still more abstract or transcendent than the efficient (nimita) or material (upadana) causes of creation. As the Gita states – Parastasmattu Bhavonyovyaktovyaktat sanatanah (VIII-20). That is the attributeless (nirguna) Brahman of Upanishads.