Every cloud… 

[1] had a natural cause 
[2] resulted from recent events
[3] was photographed in the daytime
[4] contains both solid and liquid particulates
[5] was produced by an external source of energy 

...to which the solution may be obvious... 

[1] A man-made thermonuclear explosion on earth would hardly qualify as a natural cause.
[2] A stellar "event" taking centuries to be seen on earth cannot be described as recent.
[3] A nighttime condition favors intergalactic photography on earth or spacecraft.
[4] A cloud of liquid droplets will never become formed in outer space.
[5] A cumulus tower produces its own energy day or night.

So then, does there exist any attribute shared by the clouds in all four pictures?  The title of the puzzle, To Billow or Not to Billow, offers a clue...
 

...and sure enough, all four photographs show billowing clouds.  Which is not to say that all clouds billow.  Some are clumpy globs in the sky.  Others are streaky.  Many are diffuse, with blurry edges.  A billowing cloud is recognizable at any scale much like fractals and always takes on sharply defined, convex shapes.  Billowing clouds would never be described as nebulous...
 

...and yet one of the photographs shows part of what astronomers call a nebula.  Based on dictionary definitions, the expression 'billowing nebula' would seem to be an oxymoron, like 'screaming silence'.  Centuries before the telescope, certain celestial bodies were seen to be blurred -- "smudge around the glowing fire" as described by one ancient civilization.  As telescopes have improved, nebulae have revealed themselves in exquisite detail. 

n April 22, 2010, during the celebration of the 20^th anniversary of the Hubble Space Telecope, NASA released the image below, describing it as "a pillar of gas and dust three light-years tall being eaten away by the brilliant light from nearby bright stars."   This cosmic pinnacle lies within the Carina Nebula, located 7,500 light-years away.  Not far at all, considering that the Milky Way Galaxy measures 100,000 light years in diameter.

Here on earth, a cloud must embrace at least seven conditions in order to billow....

{1} Gaseous Body ~~~~~~~~~ air, carbon dioxide, water vapor -- all are invisible, unless...
{2} Particulates ~~~ ashes, dust, smoke, soot, water droplets define the cloud visually, plus...
{3} Motion ~~~~~~~ most often convection, which requires localized expansion caused by...
{4} Heating ~~~~~~~~~~~ eruption, explosion, fire, all being external sources of energy or...
{5} Condensation ~~~ releasing latent heat of vaporization, an internal energy source, plus...
{6} Atmospheric Body ~~~~~~~~ unheated air to support convection and something else...
{7} Viscous Drag ~~ by relatively stationary air forming regions surrounded by back-flow.

Particulates of smoke or ash form clouds in billows by absorbing heat above fires or eruptions, driven upward by what is called convection.  Sophisticated solvers know that convection has an an ironic aspect -- that convection uses gravity to overcome gravity.  Warm air does not 'rise' in opposition to gravity.   Instead, heat expands air, which becomes lighter, less dense.  Unheated, unexpanded, and thus heavier air necessarily surrenders more readily to gravity, producing the requisite lifting -- think buoyancy -- to support billowing from beneath.

Water in the form of liquid particulates forms billowing clouds by convection -- not by absorbing energy but by releasing energy:  Whenever water vapor, which is invisible, gives up its 'latent heat of vaporization', water droplets form, which are comparatively opaque.  More familiar to most of us is the heat required to boil water.  Well, going the other way, that stored energy is capable of driving convection by expanding the surrounding air, with billowing clouds as the result.

We turn our attention now to the billowing clouds in the Carina Nebula and ask, How many of those seven conditions apply in interstellar space?

{1} Gaseous Body ~~~~~~~~ low pressure gases, dominated by hydrogen and helium and...
{2} Particulates ~~~~~~ dust in abundance to define a cloud visually but no water droplets...
{3} Motion ~~~~~~~~~~~~~~~~~~~~~~~~~~~ plenty of localized expansion caused by...
{4} Heating ~~~~~~~~~~~~~~~~~~~~~~~~ from natural thermonuclear explosions, but...
{5} Condensation ~~ without liquid water, no releasing of latent heat of vaporiation, and...
{6} Atmospheric Body ~~ none like terrestrial air so there must be something else for...
{7} Viscous Drag ~~ to shape convex regions surrounded by back-flow, produced by...

...a mystery!  Note the exclamation point, the only one in To Billow or Not to Billow. 

hanks to the Hubble Space Telescope, it does not take an astrophysicist to see billowing clouds in the Carina Nebula.  However, condition {7} does not appear to be satisfied.  In empty space there can be no support for Viscous Drag to induce backflows and thus to form cellular regions of interstellar clouds into convex shapes characteristic of billowing.  Unless, unless...

Dark Matter was postulated by Caltech Astronomer Fritz Zwicky about the time the author of To Billow or Not to Billow was born.  Observations relevant to Big Bang cosmology have resulted in an astounding accounting: Only 20% of the mass in the universe is made up of ordinary matter, the rest, 80%, constitutes unexplained, utterly transparent Dark Matter.  The theory continues as the most widely accepted explanation of certain anomalies in galactic motions, even though a concrete understanding of Dark Matter remains elusive. 

XMM-NEWTON FINDS MISSING INTERGALACTIC MATERIAL...

After a nearly twenty-year long game of cosmic hide-and-seek, astronomers using ESA’s XMM-Newton space observatory have finally found evidence of hot, diffuse gas permeating the cosmos, closing a puzzling gap in the overall budget of ‘normal’ matter in the Universe.