THE CLOUD
 
A quest for omicron particles turns into a potential disaster when the crew of USS Voyager discover that a giant, nebula like gas cloud is not what it seems.  Soon the ship finds itself administering an unorthodox form of medical aid. 
The Cloud is the scientifically inaccurate name given to a unique phenomenon in the Delta Quadrant.  The Cloud was discovered by the crew of the USS Voyager shortly after starting their long trip home.  The diameter of the Cloud is seven times the distance between Earth and its sun, making it larger than most star systems.  The Cloud has high levels of omicron particles which are a valuable source of energy aboard the Voyager, which is why they decided to enter the nebula(Or what they thought was a nebula due to its make up of hydrogen, helium, hydroxal radicals and dust nodules).
When the Voyager enters the cloud they find that within its mass are delicate puffs of pink matter which float freely in an ever increasing foggy haze.  In the core of the Cloud is what appears to be green globe resembling a nucleus.  The Cloud then appear to show that it is not just a dust ball,  it appears to be able to learn from experience.  It is also capable of mounting a vigorous and effective attack any any body which invades it, including the Voyager.  The intruder is covered in cloud matter and blue sphere batter the intruder zapping its energy.  Lasers and thruster even at 115% were not strong enough to free the Clouds grip on the Voyager.  The only way to escape the Cloud was to launch a photon torpedo at the wall of the Cloud causing a gapping hole to appear large enough for them to escape through.  The crew of the Voyager then discover that the Cloud is alive and the blue spheres are actually made up of organic matter and the hole they blasted in the side of the Cloud is like an open wound and the sphere's are leaking out just like blood.  The decision is made that they must repair the damage or the Cloud will die.  Because the damage is too great for the Cloud to repair itself.  They find that the Cloud responds to a nucleonic beam from the Voyager which basically sutures up the wound.  Once the wound is repaired the Voyager continues on its way.