A good question. We think it is very small, possibly lower than 1 part in a million of the matter we see. Because matter and anti-matter look absolutely identical in so far as the light they emit, we cannot tell by way of direct observation whether a particular star in the sky is made of matter of anti-matter. However, we do know that all stars in our galaxy, and all the galaxies in our corner of the universe are tied together by an interstellar medium and an intergalactic medium of stray hydrogen atoms and ionized plasma. This material comes into contact with all scraps of matter by way of a long chain of connections. If anti-matter were present in 'large' quantities such a 1 percent of all matter, or 0.001 percent, we would be able to detect the annihilation radiation at gamma ray wavelengths of electrons colliding with anti-electrons, and protons colliding with anti- protons. We do not see any large-scale evidence for this going on. When the universe was very young, there may well have been nearly equal amounts of matter and anti-matter, but virtually all of this annihilated leaving behind only a small amount of matter.
We do see a peculiar object in the center of our own Milky Way which has been named the 'Great Annihilator' because over the course of several decades, it produces a variable 511 keV gamma ray line which can only be produced if large amounts of matter and anti-matter electrons are in contact. We think this is related to something going on in the accretion disk around a massive black hole in the center of our Milky Way. The anti-matter is being produced by 'pair production' near the event horizon of the black hole.