Remarkably, it turns out there is five times more material in clusters of galaxies than we would expect from the galaxies and hot gas we can see. Most of the stuff in clusters of galaxies is invisible and, since these are the largest structures in the Universe held together by gravity, scientists then conclude that most of the matter in the entire Universe is invisible. This invisible stuff is called 'Dark Matter', a term initially coined by Fritz Zwicky who discovered evidence for missing mass in galaxies in the 1930s. There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is, how much there is, and what effect it may have on the future of the Universe as a whole.
Srimad-Bhagavatam 2.7.1 Scheduled Incarnations with Specific Functions
Lord Brahma said: When the unlimitedly powerful Lord assumed the form of a boar as a pastime, just to lift the planet earth, which was drowned in the great ocean of the universe called the Garbhodaka, the first demon [Hiranyaksa] appeared, and the Lord pierced him with His tusk.
Since the beginning of creation, the demons and the demigods, or the Vaisnavas, are always the two classes of living beings to dominate the planets of the universes. Lord Brahma is the first demigod, and Hiranyaksa is the first demon in this universe. Only under certain conditions do the planets float as weightless balls in the air, and as soon as these conditions are disturbed, the planets may fall down in the Garbhodaka Ocean, which covers half the universe. The other half is the spherical dome within which the innumerable planetary systems exist. The floating of the planets in the weightless air is due to the inner constitution of the globes, and the modernized drilling of the earth to exploit oil from within is a sort of disturbance by the modern demons and can result in a greatly harmful reaction to the floating condition of the earth. A similar disturbance was created formerly by the demons headed by Hiranyaksa (the great exploiter of the gold rush), and the earth was detached from its weightless condition and fell down into the Garbhodaka Ocean.
The Lord, as maintainer of the whole creation of the material world, therefore assumed the gigantic form of a boar with a proportionate snout and picked up the earth from within the water of Garbhodaka. Sri Jayadeva Gosvami, the great Vaisnava poet, sang as follows:
vasati dasana-sikhare dharani tava lagna
sasini kalanka-kaleva nimagna
jaya jagadisa hare
"O Kesava! O Supreme Lord who have assumed the form of a boar! O Lord! The planet earth rested on Your tusks, and it appeared like the moon engraved with spots."
Such is the symptom of an incarnation of the Lord. The incarnation of the Lord is not the concocted idea of fanciful men who create an incarnation out of imagination. The incarnation of the Lord appears under certain extraordinary circumstances like the above-mentioned occasion, and the incarnation performs a task which is not even imaginable by the tiny brain of mankind. The modern creators of the many cheap incarnations may take note of the factual incarnation of God as the gigantic boar with a suitable snout to carry the planet earth.
When the Lord appeared to pick up the earth, the demon of the name Hiranyaksa tried to create a disturbance in the methodical functions of the Lord, and therefore he was killed by being pierced by the Lord's tusk. According to Srila Jiva Gosvami, the demon Hiranyaksa was killed by the hand of the Lord. Therefore his version is that after being killed by the hand of the Lord, the demon was pierced by the tusk. Srila Visvanatha Cakravarti Thakura confirms this version.