Off topic:How to sort all the columns all together of a dataframe using R?

Question

[Paul]

I would like to sort in descending order all columns of data.frame. The dataframe looks like

mat

.      [,D1]  [,D2]  [,Dn]
[D1,]  5      1      6
[D2,]  10     3      1
[D3,]  8      8      1
[D4,]  8      9      9
[D5,]  3      7      3
[D6,]  8      8      5
[D7,]  10     10     2
[D8,]  8      10     7
[D9,]  10     1      9
[Dn,]  9      4      5

I know to sort it based on a single column by using mat <- mat[order(mat$D1),], but I have 1000 columns. How do I sort all 1000 keeping the row name same.