Despite a history that dates back at least a quarter of a century studies of voids in the large–scale structure of the Universe are bedevilled by a major problem : there exist a large number of quite different void–finding algorithms , a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense .
Because of the recent increased interest in voids , both in very large galaxy surveys and in detailed simulations of cosmic structure formation , this situation is very unfortunate .
We here present the first systematic comparison study of thirteen different void finders constructed using particles , haloes , and semi–analytical model galaxies extracted from a subvolume of the Millennium simulation .
The study includes many groups that have studied voids over the past decade .
We show their results and discuss their differences and agreements .
As it turns out , the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume .
Voids have very underdense centres , reaching below 10 percent of the mean cosmic density .
In addition , those void finders that allow for void galaxies show that those galaxies follow similar trends .
For example , the overdensity of void galaxies brighter than m _ { B } = -20 is found to be smaller than about -0.8 by all our void finding algorithms .