'Matter that just passes through each other' sounds like
another case where physicists have divided by zero.
The Milky way occupies just 100k light years and the next
decent galaxy is 2,000 K light years away ie if the galaxy represents
1cubic cm , then in a container of 20x20x20 cm or 8000cc it would be
hard to see.
So even if dark matter is 90% you only would have 10cc stuff
in a huge container of 8000cc , and if that was disperesed equally it
would be impossible to see - hence 'dark matter'. We are discovering
plenty of brown dwarfs in our own galaxy and inter galactic space
might have trillions of these planets

excerpts
What the matter is with the universe
- By Priyamvada Natarajan



There now exists incontrovertible evidence that the bulk of the matter
(85 per cent) in the universe is cold dark matter, referred to as such
as these particles do not emit any radiation in any part of the
electromagnetic spectrum.

In contrast, only four per cent of the total mass density of the
universe is composed of ordinary atoms. Very little is known about
dark matter: we do not know what it is made of, the masses of these
particles, or how and when they originate in the universe.

However, since cold dark matter has mass, it interacts gravitationally
and clusters under the attracting influence of gravity. One of the
most powerful techniques to map the distribution of dark matter in the
universe follows from Einstein's elegant theory of general relativity,
wherein he predicted the bending of light rays from distant sources
caused by the presence of mass along the line of sight to us. Just as
a concave or convex glass lens focuses light rays, the presence of all
matter in the universe causes the paths of light rays to be deflected.
This effect is referred to as gravitational lensing. Einstein
calculated that this was observable even for deflections produced by
the sun.

The seminal observation that verified this prediction occurred during
the total solar eclipse of 1919 and was observed by Arthur Eddington,
who led an expedition to South Africa to make this measurement.
Similarly, the presence of vast amounts of dark matter is inferred
from the gravitational lensing effects that they produce on the light
emitted from background galaxies.

The effect manifests itself simply - the shapes of background galaxies
seen are distorted due to the presence of foreground dark matter. As
we have prior knowledge of the undistorted and intrinsic shapes of
galaxies, we can then use the observed shapes of galaxies to infer the
properties of the intervening dark matter.

All current theoretical evidence suggests that dark matter is composed
of collisionless particles. These particles - rather than colliding
with each other - simply pass through each other. The only interaction
they feel is the gravitational attraction to each other and to
ordinary atoms.

We now have evidence from the motions of stars that most galaxies have
vast amounts of dark matter distributed in the form of an extended
halo within which the baryonic component (ordinary atoms that make up
the stars and gas) resides.

Stars in galaxies have higher speeds than would be expected if there
was no dark matter halo. Our own galaxy - the Milky Way - has a dark
halo extending to about 10^24 cm from the centre. Much of it is
distributed in the outskirts, leaving virtually no dark matter within
our solar system. Gravitational lensing deflections can be caused even
by dead stars, that eventually form black holes. These black holes
would cause the brightening of the starlight of stars moving around in
our halo.

There have been many experiments aimed at constraining such events by
continuously monitoring stars. The results of these experiments
suggest that a maximum of 20 per cent of the dark matter in the Milky
Way halo could reside in these black holes in chunks of mass weighing
between 10^{-7} to 10^{4} solar masses. So not all the dark matter in
our galaxy is attributable to these objects, a smooth dark matter
component is required to make up the rest. Besides galaxies, dark
matter is also found in copious amounts in clusters of galaxies and is
also believed to be smoothly distributed everywhere else in the
universe.

Observationally speaking, the lensing effects produced by dark matter
in clusters is spectacular (for some amazing Hubble space telescope
images, visit http://hubblesite.org) multiple images of the same
background galaxy as well as highly distorted images shaped like arcs
are detected.

Numerical simulations of the gravity-induced clustering properties of
cold dark matter are extremely well-studied. Therefore, while the
theoretical properties of cold dark matter are well understood, and
its dramatic indirect signature via gravitational lensing is seen, we
await direct detection of these particles.

Priyamvada Natarajan is associate professor, departments of astronomy
and physics, Yale University

The last word in cosmological conjecture, and perhaps yet another
possibility for cosmic apocalypse, is the unwelcome prospect -
proposed in 1980 by physicists Sidney Coleman and Frank de Luccia -
that the vacuum of deep space, although void of air, might not
actually have the lowest possible energy level. If so, this socalled
false vacuum might suddenly fail or decay, hurling the universe into
the lower energy state of a true vacuum. A bubble of true vacuum would
then flash through the universe at the speed of light, instantly
obliterating everything in its path.

Of this, we'd have not a split-second's warning.

For all we know, such a vacuum bubble might be heading our way at this
moment. Even more disconcerting is the possibility, first raised by
Piet Hut and Martin Rees in their article "How Stable Is Our Vacuum?",
published in 1983 in the British journal Nature, that we might
inadvertently create a vacuum bubble in one of our giant particle
accelerators.

An event such as this might risk creating a vacuum bubble for just a
tiny fraction of a second. But that would be all it takes to set off
the vacuum decay spelling the end of the universe.

Fortunately, there's a brighter side to the dreaded spectre of a
vacuum decay, as expounded in Davies' masterful work, The Last Three
Minutes (Weidenfeld & Nicolson, 1994). Just as we might accidentally
create a true vacuum surrounded by false vacuum, so we could, perhaps
deliberately, create the converse - namely, a bubble of false vacuum
surrounded by true vacuum.

THE universe's ultimate fate hangs largely on how much matter the
cosmos contains and of what it consists. The latest satellite
measurements suggest that 4 per cent of the universe comprises
ordinary matter (the stuff of which you, I and our belongings are
made), 23 per cent dark matter (which cannot be seen but whose
gravitational effects are measurable) and 73 per cent dark energy.
(Given the preponderance of dark entities, it's no wonder that
astronomers are literally in the dark when it comes to cosmic crystal
gazing.)

Dark energy, in particular, seems to hold the key to the far future.
Although still little understood, it's believed to be a type of
anti-gravity force that, unlike gravity, repels instead of attracts
objects at great distances, and in fact gets stronger with distance.

Sometimes you wonder what any one can gain spending so much time and
money to hide and cover up all the UFO information? Why so much
ridicule around something every country in running after since 1890?
The answer is the fact that any one who can master this time and space
bending technology will be ahead of others by many years.

There are early indications that scientists and engineers have got the
clue to the concept of bending time and space using dark energy.
Interestingly, the whole concept starts with Einstein's Theory of
Relativity.

'Matter that just passes through each other' sounds like
another case where physicists have divided by zero.