I heard this in a video, and it sounds awesome in the sense that it aligns what modern cosmology says about the contents of the universe. I'd like to know if there is an actual source in the vedic literature about this.
Yes, the universe is only one fourth manifestation of the Supreme Brahman.:
According to Ekakshara Upanishad.:
“ekaakshara.n tvakshare.atraasti some sushhumnaayaa.n cheha dR^iDhii sa ekaH |
tva.n vishvabhuurbhuutapatiH puraaNaH parjanya eko bhuvanasya goptaa |” (Ekakshara Upanishad 1)
“praaNaH prasuutirbhuvanasya yonirvyaapta.n tvayaa ekapadena vishvam.h |
tva.n vishvabhuuryonipaaraH svagarbhe kumaara eko vishikhaH sudhanvaa |” (Ekakshara Upanishad 3)
“Thou art the one Imperishable in the Imperishable, conjoint with Uma (Lord Shiva). As known by means of Susumna, here (on the empirical plane), the one firm (Principle art Thou). Thou art the ancient source of the world, the Lord of beings; Thou the Parjanya (the Principle of life-giving water), the Protector of the world. Thou art the Principle of life; Thou the manifestation (the manifested world); Thou the source of the world; by a quarter hast Thou pervaded this world. Thou art the world’s birth, the cause, the life supreme, and the child in the womb armed with the excellent bow and arrow.
This is the same thing stated in Purusha Suktam of Rig Veda as follows.:
“tripādūrdhva udait puruṣaḥ pādo.asyehābhavat punaḥ | tato viṣvaṃ vyakrāmat sāśanānaśane abhi |” (Rig Veda 10:90:4)
“Three quarters of his are beyond all this; all of this creation is but from one quarter of him. Again and again, all that eats, and that eats not appeared from this one quarter of His.”
Even the Skhambha Suktam which is a hymn on Lord Shiva’s Jwala-Linga states the same.
“yátra skambháḥ prajanáyan purāṇáṃ vyávartayat | ékaṃ tád áṅgaṃ skambhásya purāṇám anusáṃviduḥ |” (Atharva Veda 10:7:26)
“Where Skambha (Agni-Lingam) generating gave the Ancient World its shape and form, They recognized that single part of Skambha (Linga) as the Ancient World.”
I hope this clarifies your queries.
There is a verse that says that the universe is only one fourth of Brahman.
Such is the greatness of this (Brahman called Gayatri). This Person is even greater than this. All this world is a quarter of Him, the other three quarters of His (constitute) immortality in heaven.
Chandogya Upanishad 3.12.6
There is no division in Brahman; but to make it clear that compared to the phenomenal world Brahman is infinite, the parts are assumed and it is said that Brahman emanates only in one part whereas in the other three parts It is immortal or unchangeable.
Commentary by Swami Swahananda