who want to become a scientist ?if then how can you describe the fabric of space time
Answers
The "fabric" of space time is a concept from General Relativity, which says that gravity is a manifestation of warping the fabric of space-time. All mass, every galaxy, star, planet, even you and me, create little depressions in space-time. The bigger the mass, the bigger the depression.
When objects move, they move the way a marble would move, feeling the depressions and being bent towards objects. So they move in what appears to be "curved" paths in space, but really they are moving in the straightest possible path in space-time (called geodesics in GR, even though it is not necessarily related to "geo"=Earth).
The reason for including "time" in space-time is that Special Relativity tells us that a moving frame experiences time dilation. Hence when objects' motions are affected by the acceleration due to warps in space time, their clocks are also affected. Thus the distortions are not just in space, but in time as well.
Dark matter has mass, so it affects space-time like any mass. It is the dominant form of mass in the Universe, so it actually establishes the warps of space-time on large scales (i.e. tens of thousands of light years and larger). Dark matter cannot (so far as we know) condense into small objects like stars. So it doesn't generate local perturbations in things like the Solar System, so we can't see its gravitational effects on Earth, but we see that it binds together galaxies into groups and clusters.
As far as we know, dark matter only interacts gravitationally, so what you are saying that it "only" causes warps in space-time is absolutely correct. The reason we think it is actually mass and not just some unknown way in which space-time warps is that it behaves like a mass -- specifically, it gathers around objects that we can see that have mass. This can be seen by gravitational lensing, i.e. the bending of light as it passes by a collection of dark matter, in the image below it is a cluster of galaxies with dark matter (inferred from lensing) shown in blue.
Cosmic Expansion is basically just the stretching of the sheet. Imagine that you take a rubber sheet, mark it with grid lines, hold it at four corners, and pull outwards. The grid lines will just expand. If you put little marbles to represent masses, they will make depressions, but the depressions will just be "carried along" by space-time. Objects will not see themselves as "moving", yet they will see all other objects expanding away. This is what Hubble inferred when he first detected the expansion of the Universe in the 1920's, and resulted in the Hubble law .
The key is, something has to be "driving" the expansion. That is, some energy is required to actually be stretching the sheet. This energy was initially provided by the Big Bang. The remarkable discovery about 15 years ago was that the expansion is accelerating. This is strange, because as you know from stretching a sheet (or rubber band), the more you stretch, the harder it becomes to stretch more. In space-time language, the mass within the Universe itself should be slowing its own expansion. So the observable fact that the expansion is accelerating requires yet another unknown source of energy. We call this Dark Energy.
Black holes create very deep depressions in the fabric of space-time. In fact, so far as we know, it is a bottomless well. But far from the black hole, the depression in space is time is no greater than it would be any other equivalent mass! In other words, a black hole of mass M does not "suck" any more than a galaxy of mass M or a star of mass M or a bag of feathers of mass M or whatever. So it is not correct to say that black holes will suck up stuff any more than anything else. Our Milky Way has a black hole in its center that is 4 million times as massive as the Sun, but it is so far away that it adds an insignificant amount of gravity to the Earth -- we don't feel it at all. The only difference between a black hole's warp in space-time and a planets is that, if you land on the surface of the planet, you can launch yourself back out, whereas if you get sucked into a black hole, there is no coming back out!
Explosions like supernovae can cause "ripples" in space-time, because the fabric changes rapidly at one point. It is like throwing a stone into a pond. Supernova themselves come from stars, which are not very massive compared to eg supermassive black holes and don't make deep depressions in space time to begin with, so they hardly create any waves. But if you merge two galaxies with central black holes, like for instance what will happen with the Milky Way and Andromeda galaxies in about 5 billion years, then the merging black holes will create significant ripples in space-time. We are now building instruments such as LIGO to detect these sorts of gravitational waves.