When you look up into the sky at night, chances are you'll see some stars and planets glowing brightly, or you've seen the pretty pictures released by NASA every couple of weeks. For thousands of years, we've known that something was up there, producing light. However, recent studies have determined that there must be something else.
The force that gravity applies on an object is dependent on its mass. Generally, we call that the object's weight. A feather doesn't weigh much because it has very little mass, so gravity doesn't pull as hard. A rock generally weighs more, because it has more mass, and so gravity pulls more. In our solar system, the force of gravity is what keeps Earth on its orbit around the sun and makes galaxies all throughout the universe. However, we can measure the mass of objects throughout the galaxies, and we can measure the force of gravity in those galaxies, the speed at which the objects move, and it doesn't work out! Given a certain force of gravity on all the stars and planets in a galaxy, and the speed at which the revolve around, we can figure out how much mass should be in that galaxy...and it's much more than we can see. In our own Milky Way, for example, there should be about twice as much mass as there is in all the objects that we can see.
There can't just be millions of kilograms of mass missing, though. It has to be somewhere. That's how scientists came up with the idea of dark matter. We can't see it, even with the most advanced telescopes today, so it is different, and dark. However, it has mass, and therefore explains the larger than expected forces of gravity.
So what is weak lensing?
Light is a very odd thing. In a condensed version: space is curved. It's a lot like a big sheet with different sized balls on it. The big balls pull down more than the little ones. Light, being the weird thing that it is, will sometimes follow these curves, and so some of the light will go around an object. However, this light is slightly distorted from what it was before, because it may have changed directions. When only part of a light source is behind an object like that, then only part of it will be distorted. When the whole source is behind an object, the whole image will be distorted, and it may even look like there are two of that image.
When astronomers look at galaxies, they sometimes find such distortions in the image, even though it looks like nothing is there to distort it. This is because there is dark matter between the observer and the original star or galaxy. Some of the light has to go around the dark matter, and so it comes out looking different than it did before, usually more elliptical than it first did.
Most of these changes aren't very large. The object may be only 1 or 2% more elliptical than it started out. While that's hard to tell just by looking at one object, when the astronomers look at hundreds of objects, they can tell how much the light was distorted, and therefore give a guess as to how much dark matter there is between the original stars and the telescope.
The effect that dark matter (shown here in red) can produce on the images of galaxies (shown in blue).
This is weak lensing, also known as weak gravitational lensing. It's weak, because the changes are very small. Gravitational lensing refers to the fact that it was this dark matter, which affects the gravity all around, that bent the light and distorted the image of the object.
