"The dark matter really does look like matter. It does not look like a modification of gravity."
In physics, gravity, also known as gravitation or a gravitational interaction, is a fundamental interaction, which may be described as the force that draws material objects towards each other.
"Laplace made the assumption that the transmission of gravity is not instantaneous, and he found that in order to produce the known effects in the secular acceleration of the moon, gravity must travel seven million times faster than the speed of light. ...Laplaces calculation has been found to be incomplete, and his velocity of gravity illusory."
"Einsteins law of gravitation controls a geometrical quantity curvature in contrast to Newtons law which controls a mechanical quantity of force."
"One other test of gravity... is the question of whether the pull is exactly proportional to the mass... and changes in velocity are inversely proportional to the mass... That means that two objects of different mass will change their velocity in the same manner in a gravitational field. ...That is Galileos old experiment from the Leaning Tower of Pisa. ...How accurate is it? It was measured in an experiment by ...Eötvös in 1909 and ...by Dicke, and is known to one part in 10,000,000,000. ...[S]uppose you wanted to know whether the pull is exactly proportional to the inertia. The earth is going around the sun, so the things are thrown out by inertia. But they are attracted by the sun to the extent that they have ... So if they are attracted to the sun in a different proportion from that thrown out by inertia, one will be pulled towards the sun, and the other away from it, and so, hanging them on opposite ends of a rod on another Cavendish quartz fiber, the thing will twist towards the sun. It does not twist at this accuracy, so we know that the suns attraction to two objects is exactly proportional to its coefficient of inertia; in other words, its mass."
"In order that a pendulum may continue to make the same number of oscillations in a given time, it must be shortened as it is carried towards the equator; and the variation of its length in different latitudes affords an accurate measurement of the force of gravity. But the force of gravity has known relation to the figure of the earth, which, therefore, may be determined, by observing the length of the seconds pendulum at different points on its surface."
"Many other powers in nature, have an analogy to gravity, but extend to less distances, and observe laws somewhat different. It has been found very difficult to account for them mechanically. For this purpose, some have imagined certain effluvia to proceed from bodies, or atmospheres environing them; others have invented vortices; but all their attempts have hitherto proved unsatisfactory. That such powers take place in nature, and contribute to produce its chief phænomena, is most evident; but their causes are very obscure, and hardly accessible by us. In all the cases when bodies seem to act upon each other at a distance, and tend towards one another without any apparent cause impelling them, this force has been commonly called attraction and this term is frequently used by Sir Isaac Newton. But he gives repeated cautions that he pretends not, by the use of this term, to define the nature of the power, or the manner in which it acts. Nor does he ever affirm, or insinuate, that a body can act upon another at a distance, but by the intervention of other bodies. It is of the utmost importance in philosophy to establish a few general powers in nature, upon unquestionable evidence, to determine their laws, and trace their consequences, however obscure the causes of those powers may be; and this he has done with great success."
"You sometimes speak of Gravity as essential and inherent to Matter. Pray do not ascribe that Notion to me; for the Cause of Gravity is what I do not pretend to know, and therefore would take more Time to consider it."