The answer may vary between different scientists. But in my opinion, we know quite a bit about gravity. Hundreds of years ago, Newton's theory of universal gravity already captured some key features of gravity under two conditions:
(1) the objects are moving slowly (compared with the speed of light), and
(2) the gravity is not strong. Newton's was able to explain the detailed motion of planets within the Solar system.

Then about 100 years ago, Einstein revolutionized the theory of gravity, and proposed the General Relativity as a new theory of gravity, which applies even with high speed and strong gravity. In fact, Einstein's theory is so successful that it is confirmed over and over again by experiments, including the observation of gravitational waves in recent years. The gravitational wave is a prediction by Einstein's theory which does not exist in Newton's theory.

However, there is a third catch (the only remaining catch) that neither Newton nor Einstein's theory works:
(3) at very small length scale (short distance), when everything is supposedly governed by quantum mechanics. The combination of quantum mechanics and Einstein's gravity theory has puzzled scientists for nearly 100 years. But the Einstein's theory only fails at extremely small length scale, too small to "make a difference". In fact, even the most advanced lab in the world cannot detect such small length scale. So this is a question for pure theoretical physicists; for application of gravity theory in experiments or space engineering, we pretty much never have to worry about this "little annoying" incompatibility between quantum and gravity.

Gravity is one of the 4 fundamental forces, which are the forces that produce all other interactions and cannot themselves be broken down to other interactions. It is essentially the attraction between objects with masses, and therefore one might say that mass is the "cause" of gravity.

All objects in the universe which have mass are attracted to all other objects with mass (so, all matter is attracted to all other matter). Although gravity is the weakest of the fundamental forces at small scales, it is the most important for large (i.e., astronomical) distances because (i) it has infinite range (meaning it never goes away even at very large separation distances) and (ii) unlike electric charge, there is no "negative gravity" to repel bodies and cancel it out so it is always pulling all matter together. This means that gravity is responsible for the large-scale arrangement of the matter in the universe, such as galaxies and stars. It also holds Earth and the other planets around the Sun.

The strength of attraction between two objects can be calculated to a very good approximation (in many cases) using Newton's law of universal gravitation:
F = G*m₁*m₂/r²
where G is the gravitational constant (~6.674*10^-11 m³/(kg-s²)), m₁ and m₂ are the masses of the two objects, and r is the distance between their centers of mass.

Scientists also know that Newton's version of gravity is not quite right - for example, it could not explain some of the observed motion of the planet Mercury. A more accurate description uses Einstein's general relativity and leads to gravity being not so much a force as simply a consequence of the way objects move through bent spacetime (the "fabric" of the universe). (A bit more advanced, but note that some "massless" particles such as photons are also acted on by gravity - this also has to do with the curving of spacetime described in general relativity.)

We have plenty of other information about gravity as well, such as its speed (the same as the speed of light), that it travels as waves (first direct observation of this was by LIGO a few years ago), and that the current theory is possibly wrong (because there are observations which do not fit the current accepted model).

Understanding gravity is still one of the great pursuits of fundamental physics to this day.

Scientists know that gravity is a fundamental attraction between objects that have mass or energy. Basically, gravity tries to pull things towards one another. It has an infinite range but the strength of the attraction decreases strongly as a function of distance. On Earth, we feel gravity because our bodies are made up of mass and energy that is attracted to the giant ball of mass and energy that is our planet. If you watch videos of astronauts in space, you will see that they can float around. That is because they are further away from the planet, meaning that the force of gravity is weaker, so much so that they are effectively weightless. The strength of gravity on other planets is also different because the planet is of different size.

Physicists like Einstein have even shown that gravity is intimately related to time and space. If you want to learn more on that, I would encourage you to read about special and general relativity.

Scientists have made enormous leaps in understanding what gravity does and how it behaves, but still have very little understanding of exactly what causes gravity. This lack of understanding of the origins of gravity is one of the largest unanswered questions in physics, and one of the biggest problems in the standard model of particle physics.

1. What causes the force of gravity on Earth? Earth's gravity comes from all its mass. All its mass makes a combined gravitational pull on all the mass in your body. That's what gives you weight. And if you were on a planet with less mass than Earth, you would weigh less than you do here.

2. What do scientists know about the force of gravity? Sir Isaac Newton (1642 -- 1727) discovered that a force is required to change the speed or direction of movement of an object. ... Newton's "law" of gravity is a mathematical description of the way bodies are observed to attract one another, based on many scientific experiments and observations.