Is there any Dark Galaxies in the universe?. If they are present what are they?Of what matter they are formed? How to detect the dark galaxies in the universe?
Professor Pranab Kumar Bhattacharya, Rupak Bhattacharya, Ritwik Bhattacharya and Miss Upasana Bhattacharya
www.unipathos.com

There is actually one suspected in the Virgo Cluster of galaxies if I remember correctly. It is obviously invisible to the visible light, but it is visible to radio frequencies that can detect neutral hydrogen. It is believed that dark galaxies such as itconsist of dark matter and small amounts of hydrogen gas. Having lots of dark matter is not unusual, most if not all galaxies consist mostly of it, but for some reason star birth does not start in such galaxies.

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You are RightKullat Nunu. In the year 2001 Davis Disnes and his colligues detected VIRG0H121 in the virgo culster about 5 million light years away- A roatating disk galaxy that has snatched Hydrogen from a near by galaxy called NGC 4254
The big Question is does a Dark Galaxy really exist? Devoid of Light and Gas? The Question is associated with Understanding How the Universe Bloosomed from the Big Bang as we consider
Professor Pranab Kumar Bhattacharya, Rupak Bhattacharya, Ritwik Bhattacharya and Miss Upasana Bhattacharya, www.unipathos.com

Dark Galaxies
Does a dark galaxy exist? The question is associated with understanding how did the universe blossomed from the Big bang. According to increasingly refined story 85% of the matter in the universe is not ordinary baryonic matter-that makes up galaxies and stars and planets. Rather it is dark matter. As the universe grew from its infancy, the dark matter condensed in to enormous filaments like tubes, clumps and haloes. These weighty objects pooled in hydrogen gas which formed the galaxies and stars. Simulations show that dark matter should have myriad clumps between 1/1000 and 1/1000,000 as massive as our milky way galaxy. At first these small haloes should have accumulated gas and lit up as small dwarf galaxies thousands of which should whiz around Milky Way. So far astronomy could have few near by. Various factors kept the small halos dark. So space time should have many dark galaxies. They are Invisible galaxies
Professor Pranab Kumar Bhattacharya, Rupak Bhattacharya, Ritwik Bhattacharya and Miss Upasana Bhattacharya,
www.unipathos.com

Depends on your definition of "dark galaxy".

Merely hydrogen? January 2006 Universe Today: Dark Matter Galaxy?

Original PPARC news release: New evidence for a Dark Matter Galaxy

Dark matter? February 2005 Universe Today: First Dark Matter Galaxy Discovered

Original Jodrell Bank Observatory press release: Seeing the invisible - first dark galaxy discovered?

Seems like the latest news was that that dark matter was hydrogen.

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VIRGOHI21 was first described in Minchin, et al., 2005. Even in that first paper the authors pointed out that the object could be a massive HI cloud trapped in the gravity well of a dark matter cluster, or it could be tidal debris from interacting galaxies in the Virgo cluster. Bekki, Koribalski & Kilborn, 2005 numerically modeled HI tidal debris in the Virgo environment and conclude that VIRGOHI21 is most likely tidal debris associated with M99 (NGC 4254). This conclusion is reinforced by Duc & Bournaud, 2008, who are able to reproduce the details of the shape & kinematics of VIRGOHI21 as tidal debris associated with NGC 4254. At this point I think the weight of evidence shows that VIRGOHI21 is likely not a "dark galaxy", but rather a massive tidal debris gas cloud produced in the active galaxy cluster.

Meanwhile, Davies, et al., 2006 point out that current HI surveys are not sensitive enough, and don't have the velocity resolution required, to detect as many "dark galaxies" as predicted by cold dark matter models. So we are obliged to await better data.

And I would like to point out that the "dark galaxies" are not actually "galaxies" in any common sense of the word. The expectation is that there should be dark matter concentrations into which normal gas fell, but not enough to form stars, or any kind of galactic objects. Hence, a lonely cloud of neutral hydrogen gas (HI) would be expected, VIRGOHI21 being a possible example. But since such objects could also form as tidal debris, especially in a dynamic environment like the Virgo cluster, where galaxy interactions are expected, one must first show that tidal debris is an unreliable option, before drawing the more exotic conclusion of "dark galaxy".

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(bold mine)

It may be that 'dark matter' made up of neutral hydrogen (and perhaps other dust and gas) may be nothing more than 'dark mass' of the same matter detected in such small amounts. This would translate into the detected dark gas, such as neutral hydrogen, responsive to radio but not visible light, is very difuse matter of a much higher gravity G factor. This high G renders the matter gravitationally observable as high mass, i.e., 'dark mass' which we have come to call 'dark matter'. The same is present on the periphery of galaxies causing flat rotation curves. Galaxies made up of this 'high dark mass' can remain low luminosity, or virtually no luminosity, except detected by radio waves or gravitational lensing.

However, this 'dark mass' idea is not current in modern cosmology, and would need to be tested further if it is to form a working hypothesis. For now, it is merely known as 'dark matter' without further explanation, except it is believed to be 'exotic' and non-baryonic matter, which as yet remains illusive and undetected. The presence of neutral hydrogen may be a clue, however.

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Excellent reply, Tim. I just wish the popular press got a hold of the papers refuting the idea that VIRGOHI21 is a dark galaxy.