The story told by the Millennium Simulationis a spectacular one in which the very large is a consequence of theunimaginably small. The universe starts as a blazing fireball manytimes smaller than an atom and we can understand its structure byimagining an expanding balloon covered with spray paint. As the balloonexpands, the paint cracks. Something similar happened early in thehistory of the universe: it wasn't born perfect. According to Frenk,the baby universe was born with a 'rash'. No one is quite sure why thishappened but according to MIT physicist Max Tegmark: One of thefoundations of quantum physics, the laws that govern the micro-world,is the so-called Heisenberg Uncertainty Principle, which basically saysin plain English that you can't have something completely uniform. Andtherefore, early on, when everything was tiny, there must have beenthese fluctuations.Asthe universe stretches from a size smaller than an atom to thousandsand then billions of miles across, these tears in its fabric are alsostretched - think of the spray paint tearing ever wider on thatballoon. After a few million years, the universe has become a dark,freezing place full of clouds of hydrogen and helium - plus a greatdeal of that mysterious dark matter. All this matter is spread acrossthe universe in an uneven way by the primordial tears created by theBig Bang. It is at this moment that the dark matter really comes intoits own.
Its powerful gravitational pull starts sculpting the gas ofordinary atoms into what scientists call the cosmic web: it's thisamazing process that is so beautifully shown by the MillenniumSimulation. Inside the web, 'halos' of dark matter draw the gas intotighter and tighter clumps. The atoms of hydrogen, shoved together bythe power of the dark matter, begin to heat - and eventually ignite asthe first giant stars of the universe.The final result is this: two galaxies. One is made by a computer. The other is 'real'. Can you tell them apart?Frenk'sconclusion is upbeat: The galaxies generated in the simulation arealmost indistinguishable from the real galaxies, and the similaritybetween the simulated and the real galaxy tells us that the assumptionsthat we have put into the simulation concerning how the Universe began,concerning the identity of the dark matter and concerning the laws ofphysics, that those assumptions provide a good description of what wesee in nature. So the similarity between the virtual and the realgalaxies are a nice confirmation of our cosmological model of theUniverse.